When you invoke GCC, it normally does preprocessing, compilation, assembly and
linking. The "overall options" allow you to stop this process at an
intermediate stage. For example, the -c option says not to run the
linker. Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing. Some options control the
preprocessor and others the compiler itself. Yet other options control the
assembler and linker; most of these are not documented here, since you rarely
need to use any of them.

Most of the command line options that you can use with GCC are useful for C
programs; when an option is only useful with another language (usually C++),
the explanation says so explicitly. If the description for a particular option
does not mention a source language, you can use that option with all supported
languages.

The gcc program accepts options and file names as operands. Many options
have multi-letter names; therefore multiple single-letter options may
not be grouped: -dr is very different from -d -r.

You can mix options and other arguments. For the most part, the order you use
doesn't matter. Order does matter when you use several options of the same
kind; for example, if you specify -L more than once, the directories
are searched in the order specified.

Many options have long names starting with -f or with -W---for
example, -fmove-loop-invariants, -Wformat and so on. Most of
these have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. This manual documents only one of these two forms,
whichever one is not the default.

Compilation can involve up to four stages: preprocessing, compilation proper,
assembly and linking, always in that order. GCC is capable of preprocessing
and compiling several files either into several assembler input files, or into
one assembler input file; then each assembler input file produces an object
file, and linking combines all the object files (those newly compiled, and
those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of
compilation is done:

file.c

C source code which must be preprocessed.

file.i

C source code which should not be preprocessed.

file.ii

C++ source code which should not be preprocessed.

file.m

Objective-C source code. Note that you must link with the
libobjc library to make an Objective-C program work.

file.mi

Objective-C source code which should not be
preprocessed.

file.mm

file.M

Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.

file.mii

Objective-C++ source code which should not be
preprocessed.

file.h

C, C++, Objective-C or Objective-C++ header file to be
turned into a precompiled header.

file.cc

file.cp

file.cxx

file.cpp

file.CPP

file.c++

file.C

C++ source code which must be preprocessed. Note that in
.cxx, the last two letters must both be literally x.
Likewise, .C refers to a literal capital C.

file.mm

file.M

Objective-C++ source code which must be preprocessed.

file.mii

Objective-C++ source code which should not be
preprocessed.

file.hh

file.H

C++ header file to be turned into a precompiled
header.

file.f

file.for

file.FOR

Fixed form Fortran source code which should not be
preprocessed.

file.F

file.fpp

file.FPP

Fixed form Fortran source code which must be preprocessed
(with the traditional preprocessor).

file.f90

file.f95

Free form Fortran source code which should not be
preprocessed.

file.F90

file.F95

Free form Fortran source code which must be preprocessed
(with the traditional preprocessor).

file.ads

Ada source code file which contains a library unit
declaration (a declaration of a package, subprogram, or generic, or a
generic instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also called
specs.

file.adb

Ada source code file containing a library unit body (a
subprogram or package body). Such files are also called
bodies.

file.s

Assembler code.

file.S

Assembler code which must be preprocessed.

other

An object file to be fed straight into linking. Any file
name with no recognized suffix is treated this way.

You can specify the input language explicitly with the -x option:

-xlanguage

Specify explicitly the language for the following
input files (rather than letting the compiler choose a default based on
the file name suffix). This option applies to all following input files
until the next -x option. Possible values for language are:

Turn off any specification of a language, so that
subsequent files are handled according to their file name suffixes (as
they are if -x has not been used at all).

-pass-exit-codes

Normally the gcc program will exit with the code of
1 if any phase of the compiler returns a non-success return code. If you
specify -pass-exit-codes, the gcc program will instead
return with numerically highest error produced by any phase that returned
an error indication. The C, C++, and Fortran frontends return 4, if an
internal compiler error is encountered.

If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note
that some combinations (for example, -x cpp-output -E) instruct
gcc to do nothing at all.

-c

Compile or assemble the source files, but do not link. The
linking stage simply is not done. The ultimate output is in the form of an
object file for each source file.

By default, the object file name for a source file is made by replacing the
suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are
ignored.

-S

Stop after the stage of compilation proper; do not
assemble. The output is in the form of an assembler code file for each
non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing
the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

-E

Stop after the preprocessing stage; do not run the compiler
proper. The output is in the form of preprocessed source code, which is
sent to the standard output.

Input files which don't require preprocessing are ignored.

-ofile

Place output in file file. This applies regardless
to whatever sort of output is being produced, whether it be an executable
file, an object file, an assembler file or preprocessed C code.

If -o is not specified, the default is to put an executable file in
a.out, the object file for
source.suffix in
source.o, its assembler file in
source.s, a precompiled header file in
source.suffix.gch, and all
preprocessed C source on standard output.

-v

Print (on standard error output) the commands executed to
run the stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.

-###

Like -v except the commands are not executed and all
command arguments are quoted. This is useful for shell scripts to capture
the driver-generated command lines.

-pipe

Use pipes rather than temporary files for communication
between the various stages of compilation. This fails to work on some
systems where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.

-combine

If you are compiling multiple source files, this option
tells the driver to pass all the source files to the compiler at once (for
those languages for which the compiler can handle this). This will allow
intermodule analysis (IMA) to be performed by the compiler. Currently the
only language for which this is supported is C. If you pass source files
for multiple languages to the driver, using this option, the driver will
invoke the compiler(s) that support IMA once each, passing each compiler
all the source files appropriate for it. For those languages that do not
support IMA this option will be ignored, and the compiler will be invoked
once for each source file in that language. If you use this option in
conjunction with -save-temps, the compiler will generate multiple
pre-processed files (one for each source file), but only one (combined)
.o or .s file.

--help

Print (on the standard output) a description of the command
line options understood by gcc. If the -v option is also
specified then --help will also be passed on to the various
processes invoked by gcc, so that they can display the command line
options they accept. If the -Wextra option is also specified then
command line options which have no documentation associated with them will
also be displayed.

--target-help

Print (on the standard output) a description of target
specific command line options for each tool.

--version

Display the version number and copyrights of the invoked
GCC.

@file

Read command-line options from file. The options
read are inserted in place of the original @ file option. If
file does not exist, or cannot be read, then the option will be
treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character
may be included in an option by surrounding the entire option in either
single or double quotes. Any character (including a backslash) may be
included by prefixing the character to be included with a backslash. The
file may itself contain additional @ file options; any such
options will be processed recursively.

Compiling C++ Programs

C++ source files conventionally use one of the suffixes .C, .cc,
.cpp, .CPP, .c++, .cp, or .cxx; C++ header
files often use .hh or .H; and preprocessed C++ files use the
suffix .ii. GCC recognizes files with these names and compiles them as
C++ programs even if you call the compiler the same way as for compiling C
programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a
program that calls GCC and treats .c, .h and .i files as
C++ source files instead of C source files unless -x is used, and
automatically specifies linking against the C++ library. This program is also
useful when precompiling a C header file with a .h extension for use in
C++ compilations. On many systems, g++ is also installed with the name
c++.

When you compile C++ programs, you may specify many of the same command-line
options that you use for compiling programs in any language; or command-line
options meaningful for C and related languages; or options that are meaningful
only for C++ programs.

Options Controlling C Dialect

The following options control the dialect of C (or languages derived from C,
such as C++, Objective-C and Objective-C++) that the compiler accepts:

-ansi

In C mode, support all ISO C90 programs. In C++ mode,
remove GNU extensions that conflict with ISO C++.

This turns off certain features of GCC that are incompatible with ISO C90
(when compiling C code), or of standard C++ (when compiling C++ code),
such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that
identify the type of system you are using. It also enables the undesirable
and rarely used ISO trigraph feature. For the C compiler, it disables
recognition of C++ style // comments as well as the
"inline" keyword.

The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be
included in compilations done with -ansi. Alternate predefined
macros such as "__unix__" and "__vax__" are also
available, with or without -ansi.

The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.

The macro "__STRICT_ANSI__" is predefined when the -ansi
option is used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any programs
that might use these names for other things.

Functions which would normally be built in but do not have semantics defined
by ISO C (such as "alloca" and "ffs") are not built-in
functions with -ansi is used.

-std=

Determine the language standard. This option is currently
only supported when compiling C or C++. A value for this option must be
provided; possible values are

c89

iso9899:1990

ISO C90 (same as -ansi).

iso9899:199409

ISO C90 as modified in amendment 1.

c99

c9x

iso9899:1999

iso9899:199x

ISO C99. Note that this standard is not yet fully
supported; see < http://gcc.gnu.org/gcc-4.2/c99status.html>
for more information. The names c9x and iso9899:199x are
deprecated.

gnu89

Default, ISO C90 plus GNU extensions (including some C99
features).

gnu99

gnu9x

ISO C99 plus GNU extensions. When ISO C99 is fully
implemented in GCC, this will become the default. The name gnu9x is
deprecated.

c++98

The 1998 ISO C++ standard plus amendments.

gnu++98

The same as -std=c++98 plus GNU extensions. This is
the default for C++ code.

Even when this option is not specified, you can still use some of the features
of newer standards in so far as they do not conflict with previous C
standards. For example, you may use "__restrict__" even when
-std=c99 is not specified.

The -std options specifying some version of ISO C have the same effects
as -ansi, except that features that were not in ISO C90 but are in the
specified version (for example, // comments and the "inline"
keyword in ISO C99) are not disabled.

-fgnu89-inline

The option -fgnu89-inline tells GCC to use the
traditional GNU semantics for "inline" functions when in C99
mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.

This option is accepted by GCC versions 4.1.3 and up. In GCC versions prior
to 4.3, C99 inline semantics are not supported, and thus this option is
effectively assumed to be present regardless of whether or not it is
specified; the only effect of specifying it explicitly is to disable
warnings about using inline functions in C99 mode. Likewise, the option
-fno-gnu89-inline is not supported in versions of GCC before 4.3.
It will be supported only in C99 or gnu99 mode, not in C89 or gnu89 mode.

The preprocesor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are
in effect for "inline" functions.

-aux-infofilename

Output to the given filename prototyped declarations for
all functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any language
other than C.

Besides declarations, the file indicates, in comments, the origin of each
declaration (source file and line), whether the declaration was implicit,
prototyped or unprototyped ( I, N for new or O for
old, respectively, in the first character after the line number and the
colon), and whether it came from a declaration or a definition ( C
or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the
declaration.

-fno-asm

Do not recognize "asm", "inline" or
"typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__",
"__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.

In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want
to use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode ( -std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.

-fno-builtin

-fno-builtin-function

Don't recognize built-in functions that do not begin with
__builtin_ as prefix.

GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to "alloca" may become
single instructions that adjust the stack directly, and calls to
"memcpy" may become inline copy loops. The resulting code is
often both smaller and faster, but since the function calls no longer
appear as such, you cannot set a breakpoint on those calls, nor can you
change the behavior of the functions by linking with a different library.
In addition, when a function is recognized as a built-in function, GCC may
use information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the resulting
code still contains calls to that function. For example, warnings are
given with -Wformat for bad calls to "printf", when
"printf" is built in, and "strlen" is known not to
modify global memory.

With the -fno-builtin-function option only the built-in
function function is disabled. function must not begin with
__builtin_. If a function is named this is not built-in in this
version of GCC, this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in
functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:

Assert that compilation takes place in a hosted
environment. This implies -fbuiltin. A hosted environment is one in
which the entire standard library is available, and in which
"main" has a return type of "int". Examples are nearly
everything except a kernel. This is equivalent to
-fno-freestanding.

-ffreestanding

Assert that compilation takes place in a freestanding
environment. This implies -fno-builtin. A freestanding environment
is one in which the standard library may not exist, and program startup
may not necessarily be at "main". The most obvious example is an
OS kernel. This is equivalent to -fno-hosted.

-fopenmp

Enable handling of OpenMP directives "#pragma
omp" in C/C++ and "!$omp" in Fortran. When -fopenmp
is specified, the compiler generates parallel code according to the OpenMP
Application Program Interface v2.5 <
http://www.openmp.org/>.

-fms-extensions

Accept some non-standard constructs used in Microsoft
header files.

Some cases of unnamed fields in structures and unions are only accepted with
this option.

Performs a compilation in two passes: preprocessing and
compiling. This option allows a user supplied "cc1",
"cc1plus", or "cc1obj" via the -B option. The
user supplied compilation step can then add in an additional preprocessing
step after normal preprocessing but before compiling. The default is to
use the integrated cpp (internal cpp)

The semantics of this option will change if "cc1",
"cc1plus", and "cc1obj" are merged.

-traditional

-traditional-cpp

Formerly, these options caused GCC to attempt to emulate a
pre-standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode. See the
GNU CPP manual for details.

-fcond-mismatch

Allow conditional expressions with mismatched types in the
second and third arguments. The value of such an expression is void. This
option is not supported for C++.

-funsigned-char

Let the type "char" be unsigned, like
"unsigned char".

Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed
char" by default.

Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you make
such a program work with the opposite default.

The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its
behavior is always just like one of those two.

-fsigned-char

Let the type "char" be signed, like "signed
char".

Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

These options control whether a bit-field is signed or
unsigned, when the declaration does not use either "signed" or
"unsigned". By default, such a bit-field is signed, because this
is consistent: the basic integer types such as "int" are signed
types.

Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++
programs; but you can also use most of the GNU compiler options regardless of
what language your program is in. For example, you might compile a file
"firstClass.C" like this:

g++ -g -frepo -O -c firstClass.C

In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=n

Use version n of the C++ ABI. Version 2 is the
version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the
version of the C++ ABI that first appeared in G++ 3.2. Version 0 will
always be the version that conforms most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will change as
ABI bugs are fixed.

The default is version 2.

-fno-access-control

Turn off all access checking. This switch is mainly useful
for working around bugs in the access control code.

-fcheck-new

Check that the pointer returned by "operator new"
is non-null before attempting to modify the storage allocated. This check
is normally unnecessary because the C++ standard specifies that
"operator new" will only return 0 if it is declared
throw(), in which case the compiler will
always check the return value even without this option. In all other
cases, when "operator new" has a non-empty exception
specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).

-fconserve-space

Put uninitialized or runtime-initialized global variables
into the common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile with this
flag and your program mysteriously crashes after "main()" has
completed, you may have an object that is being destroyed twice because
two definitions were merged.

This option is no longer useful on most targets, now that support has been
added for putting variables into BSS without making them common.

-ffriend-injection

Inject friend functions into the enclosing namespace, so
that they are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in the old
Annotated C++ Reference Manual, and versions of G++ before 4.1 always
worked that way. However, in ISO C++ a friend function which is not
declared in an enclosing scope can only be found using argument dependent
lookup. This option causes friends to be injected as they were in earlier
releases.

This option is for compatibility, and may be removed in a future release of
G++.

-fno-elide-constructors

The C++ standard allows an implementation to omit creating
a temporary which is only used to initialize another object of the same
type. Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.

-fno-enforce-eh-specs

Don't generate code to check for violation of exception
specifications at runtime. This option violates the C++ standard, but may
be useful for reducing code size in production builds, much like defining
NDEBUG. This does not give user code permission to throw exceptions
in violation of the exception specifications; the compiler will still
optimize based on the specifications, so throwing an unexpected exception
will result in undefined behavior.

-ffor-scope

-fno-for-scope

If -ffor-scope is specified, the scope of variables
declared in a for-init-statement is limited to the for loop
itself, as specified by the C++ standard. If -fno-for-scope is
specified, the scope of variables declared in a for-init-statement
extends to the end of the enclosing scope, as was the case in old versions
of G++, and other (traditional) implementations of C++.

The default if neither flag is given to follow the standard, but to allow
and give a warning for old-style code that would otherwise be invalid, or
have different behavior.

-fno-gnu-keywords

Do not recognize "typeof" as a keyword, so that
code can use this word as an identifier. You can use the keyword
"__typeof__" instead. -ansi implies
-fno-gnu-keywords.

-fno-implicit-templates

Never emit code for non-inline templates which are
instantiated implicitly (i.e. by use); only emit code for explicit
instantiations.

-fno-implicit-inline-templates

Don't emit code for implicit instantiations of inline
templates, either. The default is to handle inlines differently so that
compiles with and without optimization will need the same set of explicit
instantiations.

-fno-implement-inlines

To save space, do not emit out-of-line copies of inline
functions controlled by #pragma implementation. This will cause
linker errors if these functions are not inlined everywhere they are
called.

-fms-extensions

Disable pedantic warnings about constructs used in MFC,
such as implicit int and getting a pointer to member function via
non-standard syntax.

-fno-nonansi-builtins

Disable built-in declarations of functions that are not
mandated by ANSI/ISO C. These include "ffs", "alloca",
"_exit", "index", "bzero",
"conjf", and other related functions.

-fno-operator-names

Do not treat the operator name keywords "and",
"bitand", "bitor", "compl", "not",
"or" and "xor" as synonyms as keywords.

-fno-optional-diags

Disable diagnostics that the standard says a compiler does
not need to issue. Currently, the only such diagnostic issued by G++ is
the one for a name having multiple meanings within a class.

-fpermissive

Downgrade some diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive will allow some
nonconforming code to compile.

Disable generation of information about every class with
virtual functions for use by the C++ runtime type identification features
( dynamic_cast and typeid). If you don't use those parts of
the language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate it as
needed. The dynamic_cast operator can still be used for casts that
do not require runtime type information, i.e. casts to "void *"
or to unambiguous base classes.

-fstats

Emit statistics about front-end processing at the end of
the compilation. This information is generally only useful to the G++
development team.

-ftemplate-depth-n

Set the maximum instantiation depth for template classes to
n. A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17.

-fno-threadsafe-statics

Do not emit the extra code to use the routines specified in
the C++ ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need to be
thread-safe.

-fuse-cxa-atexit

Register destructors for objects with static storage
duration with the "__cxa_atexit" function rather than the
"atexit" function. This option is required for fully
standards-compliant handling of static destructors, but will only work if
your C library supports "__cxa_atexit".

-fno-use-cxa-get-exception-ptr

Don't use the "__cxa_get_exception_ptr" runtime
routine. This will cause "std::uncaught_exception" to be
incorrect, but is necessary if the runtime routine is not available.

-fvisibility-inlines-hidden

This switch declares that the user does not attempt to
compare pointers to inline methods where the addresses of the two
functions were taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with
"__attribute__ ((visibility ("hidden")))" so that they
do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can have a
dramatic effect on load and link times of a DSO as it massively reduces
the size of the dynamic export table when the library makes heavy use of
templates.

The behaviour of this switch is not quite the same as marking the methods as
hidden directly, because it does not affect static variables local to the
function or cause the compiler to deduce that the function is defined in
only one shared object.

You may mark a method as having a visibility explicitly to negate the effect
of the switch for that method. For example, if you do want to compare
pointers to a particular inline method, you might mark it as having
default visibility. Marking the enclosing class with explicit visibility
will have no effect.

Explicitly instantiated inline methods are unaffected by this option as
their linkage might otherwise cross a shared library boundary.

-fno-weak

Do not use weak symbol support, even if it is provided by
the linker. By default, G++ will use weak symbols if they are available.
This option exists only for testing, and should not be used by end-users;
it will result in inferior code and has no benefits. This option may be
removed in a future release of G++.

-nostdinc++

Do not search for header files in the standard directories
specific to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have
meanings only for C++ programs:

-fno-default-inline

Do not assume inline for functions defined inside a
class scope.
Note that these functions will have linkage like inline functions; they
just won't be inlined by default.

-Wabi (C++ only)

Warn when G++ generates code that is probably not
compatible with the vendor-neutral C++ ABI. Although an effort has been
made to warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code. There
may also be cases where warnings are emitted even though the code that is
generated will be compatible.

You should rewrite your code to avoid these warnings if you are concerned
about the fact that code generated by G++ may not be binary compatible
with code generated by other compilers.

The known incompatibilities at this point include:

*

Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For example:

In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this problem
by explicitly padding "A" so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout "B" identically.

*

Incorrect handling of tail-padding for virtual bases. G++
does not use tail padding when laying out virtual bases. For example:

In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other
compilers to layout "C" identically.

*

Incorrect handling of bit-fields with declared widths
greater than that of their underlying types, when the bit-fields appear in
a union. For example:

union U { int i : 4096; };

Assuming that an "int" does not have 4096 bits, G++ will make the
union too small by the number of bits in an "int".

*

Empty classes can be placed at incorrect offsets. For
example:

struct A {};

struct B {
A a;
virtual void f ();
};

struct C : public B, public A {};

G++ will place the "A" base class of "C" at a nonzero
offset; it should be placed at offset zero. G++ mistakenly believes that
the "A" data member of "B" is already at offset
zero.

Warn when a class seems unusable because all the
constructors or destructors in that class are private, and it has neither
friends nor public static member functions.

-Wnon-virtual-dtor (C++ only)

Warn when a class appears to be polymorphic, thereby
requiring a virtual destructor, yet it declares a non-virtual one. This
warning is also enabled if -Weffc++ is specified.

-Wreorder (C++ only)

Warn when the order of member initializers given in the
code does not match the order in which they must be executed. For
instance:

struct A {
int i;
int j;
A(): j (0), i (1) { }
};

The compiler will rearrange the member initializers for i and
j to match the declaration order of the members, emitting a warning
to that effect. This warning is enabled by -Wall.

The following -W... options are not affected by -Wall.

-Weffc++ (C++ only)

Warn about violations of the following style guidelines
from Scott Meyers' Effective C++ book:

*

Item 11: Define a copy constructor and an assignment
operator for classes with dynamically allocated memory.

*

Item 12: Prefer initialization to assignment in
constructors.

*

Item 14: Make destructors virtual in base classes.

*

Item 15: Have "operator=" return a reference to
*this.

*

Item 23: Don't try to return a reference when you must
return an object.

Also warn about violations of the following style guidelines from Scott Meyers'
More Effective C++ book:

*

Item 6: Distinguish between prefix and postfix forms of
increment and decrement operators.

*

Item 7: Never overload "&&",
"⎪⎪", or ",".

When selecting this option, be aware that the standard library headers do not
obey all of these guidelines; use grep -v to filter out those
warnings.

-Wno-deprecated (C++ only)

Do not warn about usage of deprecated features.

-Wstrict-null-sentinel (C++ only)

Warn also about the use of an uncasted "NULL" as
sentinel. When compiling only with GCC this is a valid sentinel, as
"NULL" is defined to "__null". Although it is a null
pointer constant not a null pointer, it is guaranteed to of the same size
as a pointer. But this use is not portable across different
compilers.

-Wno-non-template-friend (C++ only)

Disable warnings when non-templatized friend functions are
declared within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language
specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit
specification, unqualified-ids could be interpreted as a particular
specialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++,
-Wnon-template-friend allows the compiler to check existing code
for potential trouble spots and is on by default. This new compiler
behavior can be turned off with -Wno-non-template-friend which
keeps the conformant compiler code but disables the helpful warning.

-Wold-style-cast (C++ only)

Warn if an old-style (C-style) cast to a non-void type is
used within a C++ program. The new-style casts ( dynamic_cast,
static_cast, reinterpret_cast, and const_cast) are
less vulnerable to unintended effects and much easier to search for.

-Woverloaded-virtual (C++ only)

Warn when a function declaration hides virtual functions
from a base class. For example, in:

struct A {
virtual void f();
};

struct B: public A {
void f(int);
};

the "A" class version of "f" is hidden in "B",
and code like:

B* b;
b->f();

will fail to compile.

-Wno-pmf-conversions (C++ only)

Disable the diagnostic for converting a bound pointer to
member function to a plain pointer.

-Wsign-promo (C++ only)

Warn when overload resolution chooses a promotion from
unsigned or enumerated type to a signed type, over a conversion to an
unsigned type of the same size. Previous versions of G++ would try to
preserve unsignedness, but the standard mandates the current behavior.

struct A {
operator int ();
A& operator = (int);
};

main ()
{
A a,b;
a = b;
}

In this example, G++ will synthesize a default A& operator =(const A&);, while cfront will use the user-defined operator
=.

Options Controlling Objective-C and Objective-C++ Dialects

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages
themselves. See

This section describes the command-line options that are only meaningful for
Objective-C and Objective-C++ programs, but you can also use most of the
language-independent GNU compiler options. For example, you might compile a
file "some_class.m" like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any language
supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:

-fconstant-string-class=class-name

Use class-name as the name of the class to
instantiate for each literal string specified with the syntax
"@"..."". The default class name is
"NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present, will
override the -fconstant-string-class setting and cause
"@"..."" literals to be laid out as constant
CoreFoundation strings.

-fgnu-runtime

Generate object code compatible with the standard GNU
Objective-C runtime. This is the default for most types of systems.

-fnext-runtime

Generate output compatible with the NeXT runtime. This is
the default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.

-fno-nil-receivers

Assume that all Objective-C message dispatches (e.g.,
"[receiver message:arg]") in this translation unit ensure that
the receiver is not "nil". This allows for more efficient entry
points in the runtime to be used. Currently, this option is only available
in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fobjc-call-cxx-cdtors

For each Objective-C class, check if any of its instance
variables is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method
that will run non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial destructor, and
if so, synthesize a special "- (void) .cxx_destruct" method that
will run all such default destructors, in reverse order.

The "- (id) .cxx_construct" and/or "- (void)
.cxx_destruct" methods thusly generated will only operate on instance
variables declared in the current Objective-C class, and not those
inherited from superclasses. It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods will be invoked by the
runtime immediately after a new object instance is allocated; the "-
(void) .cxx_destruct" methods will be invoked immediately before the
runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.

-fobjc-direct-dispatch

Allow fast jumps to the message dispatcher. On Darwin this
is accomplished via the comm page.

-fobjc-exceptions

Enable syntactic support for structured exception handling
in Objective-C, similar to what is offered by C++ and Java. This option is
unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and
earlier.

The @throw statement may appear anywhere in an Objective-C or Objective-C++
program; when used inside of a @catch block, the @throw may appear without
an argument (as shown above), in which case the object caught by the
@catch will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and caught
using this scheme. When an object is thrown, it will be caught by the
nearest @catch clause capable of handling objects of that type,
analogously to how "catch" blocks work in C++ and Java. A
"@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous @catch
clauses (if any).

The @finally clause, if present, will be executed upon exit from the
immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or rethrown
inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.

There are several caveats to using the new exception mechanism:

*

Although currently designed to be binary compatible with
"NS_HANDLER"-style idioms provided by the
"NSException" class, the new exceptions can only be used on Mac
OS X 10.3 (Panther) and later systems, due to additional functionality
needed in the (NeXT) Objective-C runtime.

*

As mentioned above, the new exceptions do not support
handling types other than Objective-C objects. Furthermore, when used from
Objective-C++, the Objective-C exception model does not interoperate with
C++ exceptions at this time. This means you cannot @throw an exception
from Objective-C and "catch" it in C++, or vice versa (i.e.,
"throw ... @catch").

The -fobjc-exceptions switch also enables the use of synchronization
blocks for thread-safe execution:

@synchronized (ObjCClass *guard) {
...
}

Upon entering the @synchronized block, a thread of execution shall first check
whether a lock has been placed on the corresponding "guard" object
by another thread. If it has, the current thread shall wait until the other
thread relinquishes its lock. Once "guard" becomes available, the
current thread will place its own lock on it, execute the code contained in
the @synchronized block, and finally relinquish the lock (thereby making
"guard" available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked
@synchronized. Note that throwing exceptions out of @synchronized blocks is
allowed, and will cause the guarding object to be unlocked properly.

Emit a special marker instructing
ld(1) not to statically link in the resulting
object file, and allow dyld(1) to load it in
at run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself. Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.

-fzero-link

When compiling for the NeXT runtime, the compiler
ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time) with static class
references that get initialized at load time, which improves run-time
performance. Specifying the -fzero-link flag suppresses this
behavior and causes calls to "objc_getClass("...")" to
be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program
execution.

-gen-decls

Dump interface declarations for all classes seen in the
source file to a file named sourcename.decl.

-Wassign-intercept

Warn whenever an Objective-C assignment is being
intercepted by the garbage collector.

-Wno-protocol

If a class is declared to implement a protocol, a warning
is issued for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method not
explicitly implemented in the class, even if a method implementation is
inherited from the superclass. If you use the -Wno-protocol option,
then methods inherited from the superclass are considered to be
implemented, and no warning is issued for them.

-Wselector

Warn if multiple methods of different types for the same
selector are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a check is
performed for each selector appearing in a "@selector(...)"
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at the
end of compilation, these warnings are not produced if the final stage of
compilation is not reached, for example because an error is found during
compilation, or because the -fsyntax-only option is being
used.

-Wstrict-selector-match

Warn if multiple methods with differing argument and/or
return types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or
"Class". When this flag is off (which is the default behavior),
the compiler will omit such warnings if any differences found are confined
to types which share the same size and alignment.

-Wundeclared-selector

Warn if a "@selector(...)" expression referring
to an undeclared selector is found. A selector is considered undeclared if
no method with that name has been declared before the
"@selector(...)" expression, either explicitly in an @interface
or @protocol declaration, or implicitly in an @implementation section.
This option always performs its checks as soon as a
"@selector(...)" expression is found, while -Wselector
only performs its checks in the final stage of compilation. This also
enforces the coding style convention that methods and selectors must be
declared before being used.

-print-objc-runtime-info

Generate C header describing the largest structure that is
passed by value, if any.

Options to Control Diagnostic Messages Formatting

Traditionally, diagnostic messages have been formatted irrespective of the
output device's aspect (e.g. its width, ...). The options described below can
be used to control the diagnostic messages formatting algorithm, e.g. how many
characters per line, how often source location information should be reported.
Right now, only the C++ front end can honor these options. However it is
expected, in the near future, that the remaining front ends would be able to
digest them correctly.

-fmessage-length=n

Try to format error messages so that they fit on lines of
about n characters. The default is 72 characters for g++ and
0 for the rest of the front ends supported by GCC. If n is zero,
then no line-wrapping will be done; each error message will appear on a
single line.

-fdiagnostics-show-location=once

Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit once source location
information; that is, in case the message is too long to fit on a single
physical line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines. This
is the default behavior.

-fdiagnostics-show-location=every-line

Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit the same source location information
(as prefix) for physical lines that result from the process of breaking a
message which is too long to fit on a single line.

-fdiagnostics-show-option

This option instructs the diagnostic machinery to add text
to each diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to the
diagnostic machinery.

Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions which are not
inherently erroneous but which are risky or suggest there may have been an
error.

You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This
manual lists only one of the two forms, whichever is not the default.

The following options control the amount and kinds of warnings produced by GCC;
for further, language-specific options also refer to C++ Dialect
Options and Objective-C and Objective-C++ DialectOptions.

-fsyntax-only

Check the code for syntax errors, but don't do anything
beyond that.

-pedantic

Issue all the warnings demanded by strict ISO C and ISO
C++; reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are
supported as well. With this option, they are rejected.

-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes;
application programs should avoid them.

Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they want: it
finds some non-ISO practices, but not all---only those for which ISO C
requires a diagnostic, and some others for which diagnostics have
been added.

A feature to report any failure to conform to ISO C might be useful in some
instances, but would require considerable additional work and would be
quite different from -pedantic. We don't have plans to support such
a feature in the near future.

Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu89 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -pedantic are given where
they are required by the base standard. (It would not make sense for such
warnings to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing to
warn about.)

-pedantic-errors

Like -pedantic, except that errors are produced
rather than warnings.

-w

Inhibit all warning messages.

-Wno-import

Inhibit warning messages about the use of
#import.

-Wchar-subscripts

Warn if an array subscript has type "char". This
is a common cause of error, as programmers often forget that this type is
signed on some machines. This warning is enabled by -Wall.

-Wcomment

Warn whenever a comment-start sequence /* appears in
a /* comment, or whenever a Backslash-Newline appears in a
// comment. This warning is enabled by -Wall.

-Wfatal-errors

This option causes the compiler to abort compilation on the
first error occurred rather than trying to keep going and printing further
error messages.

-Wformat

Check calls to "printf" and "scanf",
etc., to make sure that the arguments supplied have types appropriate to
the format string specified, and that the conversions specified in the
format string make sense. This includes standard functions, and others
specified by format attributes, in the "printf",
"scanf", "strftime" and "strfmon" (an X/Open
extension, not in the C standard) families (or other target-specific
families). Which functions are checked without format attributes having
been specified depends on the standard version selected, and such checks
of functions without the attribute specified are disabled by
-ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU libc
version 2.2. These include all ISO C90 and C99 features, as well as
features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -pedantic is used
with -Wformat, warnings will be given about format features not in
the selected standard version (but not for "strfmon" formats,
since those are not in any version of the C standard).

Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.

-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in
-Wall.

-Wformat-y2k

If -Wformat is specified, also warn about
"strftime" formats which may yield only a two-digit year.

-Wno-format-extra-args

If -Wformat is specified, do not warn about excess
arguments to a "printf" or "scanf" format function.
The C standard specifies that such arguments are ignored.

Where the unused arguments lie between used arguments that are specified
with $ operand number specifications, normally warnings are still
given, since the implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the case of
"scanf" formats, this option will suppress the warning if the
unused arguments are all pointers, since the Single Unix Specification
says that such unused arguments are allowed.

-Wno-format-zero-length

If -Wformat is specified, do not warn about
zero-length formats. The C standard specifies that zero-length formats are
allowed.

-Wformat-nonliteral

If -Wformat is specified, also warn if the format
string is not a string literal and so cannot be checked, unless the format
function takes its format arguments as a "va_list".

-Wformat-security

If -Wformat is specified, also warn about uses of
format functions that represent possible security problems. At present,
this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal and there are no
format arguments, as in "printf (foo);". This may be a security
hole if the format string came from untrusted input and contains
%n. (This is currently a subset of what -Wformat-nonliteral
warns about, but in future warnings may be added to
-Wformat-security that are not included in
-Wformat-nonliteral.)

-Wformat=2

Enable -Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat-Wformat-nonliteral -Wformat-security -Wformat-y2k.

-Wnonnull

Warn about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be
disabled with the -Wno-nonnull option.

-Winit-self (C, C++, Objective-C and Objective-C++
only)

Warn about uninitialized variables which are initialized
with themselves. Note this option can only be used with the
-Wuninitialized option, which in turn only works with -O1
and above.

For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:

int f()
{
int i = i;
return i;
}

-Wimplicit-int

Warn when a declaration does not specify a type. This
warning is enabled by -Wall.

-Wimplicit-function-declaration

-Werror-implicit-function-declaration

Give a warning (or error) whenever a function is used
before being declared. The form
-Wno-error-implicit-function-declaration is not supported. This
warning is enabled by -Wall (as a warning, not an error).

-Wimplicit

Same as -Wimplicit-int and
-Wimplicit-function-declaration. This warning is enabled by
-Wall.

-Wmain

Warn if the type of main is suspicious. main
should be a function with external linkage, returning int, taking either
zero arguments, two, or three arguments of appropriate types. This warning
is enabled by -Wall.

-Wmissing-braces

Warn if an aggregate or union initializer is not fully
bracketed. In the following example, the initializer for a is not
fully bracketed, but that for b is fully bracketed.

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

-Wmissing-include-dirs (C, C++, Objective-C and
Objective-C++ only)

Warn if a user-supplied include directory does not
exist.

-Wparentheses

Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people often get
confused about. Only the warning for an assignment used as a truth value
is supported when compiling C++; the other warnings are only supported
when compiling C.

Also warn if a comparison like x<=y<=z appears; this is
equivalent to (x<=y ? 1 : 0) <= z, which is a different
interpretation from that of ordinary mathematical notation.

Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an
example of such a case:

{
if (a)
if (b)
foo ();
else
bar ();
}

In C, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)".
This is often not what the programmer expected, as illustrated in the
above example by indentation the programmer chose. When there is the
potential for this confusion, GCC will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around the
innermost "if" statement so there is no way the "else"
could belong to the enclosing "if". The resulting code would
look like this:

{
if (a)
{
if (b)
foo ();
else
bar ();
}
}

This warning is enabled by -Wall.

-Wsequence-point

Warn about code that may have undefined semantics because
of violations of sequence point rules in the C and C++ standards.

The C and C++ standards defines the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part of
a larger expression), after the evaluation of the first operand of a
"&&", "⎪⎪", "? :" or
"," (comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed by the
sequence point rules, the order of evaluation of subexpressions of an
expression is not specified. All these rules describe only a partial order
rather than a total order, since, for example, if two functions are called
within one expression with no sequence point between them, the order in
which the functions are called is not specified. However, the standards
committee have ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the values
of objects take effect. Programs whose behavior depends on this have
undefined behavior; the C and C++ standards specify that "Between the
previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression. Furthermore, the
prior value shall be read only to determine the value to be stored.".
If a program breaks these rules, the results on any particular
implementation are entirely unpredictable.

Examples of code with undefined behavior are "a = a++;",
"a[n] = b[n++]" and "a[i++] = i;". Some more
complicated cases are not diagnosed by this option, and it may give an
occasional false positive result, but in general it has been found fairly
effective at detecting this sort of problem in programs.

The standard is worded confusingly, therefore there is some debate over the
precise meaning of the sequence point rules in subtle cases. Links to
discussions of the problem, including proposed formal definitions, may be
found on the GCC readings page, at <
http://gcc.gnu.org/readings.html>.

This warning is enabled by -Wall for C and C++.

-Wreturn-type

Warn whenever a function is defined with a return-type that
defaults to "int". Also warn about any "return"
statement with no return-value in a function whose return-type is not
"void".

For C, also warn if the return type of a function has a type qualifier such
as "const". Such a type qualifier has no effect, since the value
returned by a function is not an lvalue. ISO C prohibits qualified
"void" return types on function definitions, so such return
types always receive a warning even without this option.

For C++, a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.

This warning is enabled by -Wall.

-Wswitch

Warn whenever a "switch" statement has an index
of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. (The presence of a "default"
label prevents this warning.) "case" labels outside the
enumeration range also provoke warnings when this option is used. This
warning is enabled by -Wall.

-Wswitch-default

Warn whenever a "switch" statement does not have
a "default" case.

-Wswitch-enum

Warn whenever a "switch" statement has an index
of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. "case" labels outside the
enumeration range also provoke warnings when this option is used.

-Wtrigraphs

Warn if any trigraphs are encountered that might change the
meaning of the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.

-Wunused-function

Warn whenever a static function is declared but not defined
or a non-inline static function is unused. This warning is enabled by
-Wall.

-Wunused-label

Warn whenever a label is declared but not used. This
warning is enabled by -Wall.

To suppress this warning use the unused attribute.

-Wunused-parameter

Warn whenever a function parameter is unused aside from its
declaration.

To suppress this warning use the unused attribute.

-Wunused-variable

Warn whenever a local variable or non-constant static
variable is unused aside from its declaration. This warning is enabled by
-Wall.

To suppress this warning use the unused attribute.

-Wunused-value

Warn whenever a statement computes a result that is
explicitly not used. This warning is enabled by -Wall.

To suppress this warning cast the expression to void.

-Wunused

All the above -Wunused options combined.

In order to get a warning about an unused function parameter, you must
either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.

-Wuninitialized

Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp"
call.

These warnings are possible only in optimizing compilation, because they
require data flow information that is computed only when optimizing. If
you do not specify -O, you will not get these warnings. Instead,
GCC will issue a warning about -Wuninitialized requiring -O.

If you want to warn about code which uses the uninitialized value of the
variable in its own initializer, use the -Winit-self option.

These warnings occur for individual uninitialized or clobbered elements of
structure, union or array variables as well as for variables which are
uninitialized or clobbered as a whole. They do not occur for variables or
elements declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there are warnings
will depend on the precise optimization options and version of GCC used.

Note that there may be no warning about a variable that is used only to
compute a value that itself is never used, because such computations may
be deleted by data flow analysis before the warnings are printed.

These warnings are made optional because GCC is not smart enough to see all
the reasons why the code might be correct despite appearing to have an
error. Here is one example of how this can happen:

This option also warns when a non-volatile automatic variable might be
changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.

The compiler sees only the calls to "setjmp". It cannot know where
"longjmp" will be called; in fact, a signal handler could call
it at any point in the code. As a result, you may get a warning even when
there is in fact no problem because "longjmp" cannot in fact be
called at the place which would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you
use that never return as "noreturn".

This warning is enabled by -Wall.

-Wunknown-pragmas

Warn when a #pragma directive is encountered which is not
understood by GCC. If this command line option is used, warnings will even
be issued for unknown pragmas in system header files. This is not the case
if the warnings were only enabled by the -Wall command line
option.

-Wno-pragmas

Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.

-Wstrict-aliasing

This option is only active when -fstrict-aliasing is
active. It warns about code which might break the strict aliasing rules
that the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It is
included in -Wall.

-Wstrict-aliasing=2

This option is only active when -fstrict-aliasing is
active. It warns about code which might break the strict aliasing rules
that the compiler is using for optimization. This warning catches more
cases than -Wstrict-aliasing, but it will also give a warning for
some ambiguous cases that are safe.

-Wstrict-overflow

-Wstrict-overflow=n

This option is only active when -fstrict-overflow is
active. It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does not warn
about all cases where the code might overflow: it only warns about cases
where the compiler implements some optimization. Thus this warning depends
on the optimization level.

An optimization which assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can easily
give a false positive: a warning about code which is not actually a
problem. To help focus on important issues, several warning levels are
defined. No warnings are issued for the use of undefined signed overflow
when estimating how many iterations a loop will require, in particular
when determining whether a loop will be executed at all.

@option<-Wstrict-overflow=1>

Warn about cases which are both questionable and easy to
avoid. For example: "x + 1 > x"; with
-fstrict-overflow, the compiler will simplify this to 1. This level
of -Wstrict-overflow is enabled by -Wall; higher levels are
not, and must be explicitly requested.

@option<-Wstrict-overflow=2>

Also warn about other cases where a comparison is
simplified to a constant. For example: "abs (x) >= 0". This
can only be simplified when -fstrict-overflow is in effect, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less
than zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.

@option<-Wstrict-overflow=3>

Also warn about other cases where a comparison is
simplified. For example: "x + 1 > 1" will be simplified to
"x > 0".

@option<-Wstrict-overflow=4>

Also warn about other simplifications not covered by the
above cases. For example: "(x * 10) / 5" will be simplified to
"x * 2".

@option<-Wstrict-overflow=5>

Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison. For example: "x + 2
> y" will be simplified to "x + 1 >= y". This is
reported only at the highest warning level because this simplification
applies to many comparisons, so this warning level will give a very large
number of false positives.

-Wall

All of the above -W options combined. This enables
all the warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the
warning), even in conjunction with macros. This also enables some
language-specific warnings described in C++ Dialect Options and
Objective-C and Objective-C++ Dialect Options.

The following -W... options are not implied by -Wall. Some of them
warn about constructions that users generally do not consider questionable,
but which occasionally you might wish to check for; others warn about
constructions that are necessary or hard to avoid in some cases, and there is
no simple way to modify the code to suppress the warning.

-Wextra

(This option used to be called -W. The older name is
still supported, but the newer name is more descriptive.) Print extra
warning messages for these events:

*

A function can return either with or without a value.
(Falling off the end of the function body is considered returning without
a value.) For example, this function would evoke such a warning:

foo (a)
{
if (a > 0)
return a;
}

*

An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning, cast the
unused expression to void. For example, an expression such as
x[i,j] will cause a warning, but x[(void)i,j] will not.

*

An unsigned value is compared against zero with <
or >=.

*

Storage-class specifiers like "static" are not
the first things in a declaration. According to the C Standard, this usage
is obsolescent.

*

If -Wall or -Wunused is also specified, warn
about unused arguments.

*

A comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also
specified.)

*

An aggregate has an initializer which does not initialize
all members. This warning can be independently controlled by
-Wmissing-field-initializers.

*

An initialized field without side effects is overridden
when using designated initializers. This warning can be independently
controlled by -Woverride-init.

*

A function parameter is declared without a type specifier
in K&R-style functions:

void foo(bar) { }

*

An empty body occurs in an if or else
statement.

*

A pointer is compared against integer zero with
<, <=, >, or >=.

*

A variable might be changed by longjmp or
vfork.

*<(C++ only)>

An enumerator and a non-enumerator both appear in a
conditional expression.

*<(C++ only)>

A non-static reference or non-static const member
appears in a class without constructors.

*<(C++ only)>

Ambiguous virtual bases.

*<(C++ only)>

Subscripting an array which has been declared
register.

*<(C++ only)>

Taking the address of a variable which has been declared
register.

*<(C++ only)>

A base class is not initialized in a derived class' copy
constructor.

-Wno-div-by-zero

Do not warn about compile-time integer division by zero.
Floating point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.

-Wsystem-headers

Print warning messages for constructs found in system
header files. Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and would only
make the compiler output harder to read. Using this command line option
tells GCC to emit warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with this
option will not warn about unknown pragmas in system headers---for
that, -Wunknown-pragmas must also be used.

-Wfloat-equal

Warn if floating point values are used in equality
comparisons.

The idea behind this is that sometimes it is convenient (for the programmer)
to consider floating-point values as approximations to infinitely precise
real numbers. If you are doing this, then you need to compute (by
analyzing the code, or in some other way) the maximum or likely maximum
error that the computation introduces, and allow for it when performing
comparisons (and when producing output, but that's a different problem).
In particular, instead of testing for equality, you would check to see
whether the two values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C only)

Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and/or problematic constructs which should be
avoided.

*

Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place within string
literals, but does not in ISO C.

*

In traditional C, some preprocessor directives did not
exist. Traditional preprocessors would only consider a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C understands
but would ignore because the # does not appear as the first
character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.

*

A function-like macro that appears without arguments.

*

The unary plus operator.

*

The U integer constant suffix, or the F or
L floating point constant suffixes. (Traditional C does support the
L suffix on integer constants.) Note, these suffixes appear in
macros defined in the system headers of most modern systems, e.g. the
_MIN/ _MAX macros in "<limits.h>". Use of
these macros in user code might normally lead to spurious warnings,
however GCC's integrated preprocessor has enough context to avoid warning
in these cases.

*

A function declared external in one block and then used
after the end of the block.

*

A "switch" statement has an operand of type
"long".

*

A non-"static" function declaration follows a
"static" one. This construct is not accepted by some traditional
C compilers.

*

The ISO type of an integer constant has a different width
or signedness from its traditional type. This warning is only issued if
the base of the constant is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned about.

Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the zero
initializer in user code appears conditioned on e.g. "__STDC__"
to avoid missing initializer warnings and relies on default initialization
to zero in the traditional C case.

*

Conversions by prototypes between fixed/floating point
values and vice versa. The absence of these prototypes when compiling with
traditional C would cause serious problems. This is a subset of the
possible conversion warnings, for the full set use
-Wconversion.

*

Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or variadic
functions because these ISO C features will appear in your code when using
libiberty's traditional C compatibility macros, "PARAMS" and
"VPARAMS". This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.

-Wdeclaration-after-statement (C only)

Warn when a declaration is found after a statement in a
block. This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90 and was not
supported by GCC versions before GCC 3.0.

-Wundef

Warn if an undefined identifier is evaluated in an
#if directive.

-Wno-endif-labels

Do not warn whenever an #else or an #endif
are followed by text.

-Wshadow

Warn whenever a local variable shadows another local
variable, parameter or global variable or whenever a built-in function is
shadowed.

-Wlarger-than-len

Warn whenever an object of larger than len bytes is
defined.

-Wunsafe-loop-optimizations

Warn if the loop cannot be optimized because the compiler
could not assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler made such
assumptions.

-Wpointer-arith

Warn about anything that depends on the "size of"
a function type or of "void". GNU C assigns these types a size
of 1, for convenience in calculations with "void *" pointers and
pointers to functions.

-Wbad-function-cast (C only)

Warn whenever a function call is cast to a non-matching
type. For example, warn if "int malloc()" is cast to
"anything *".

-Wc++-compat

Warn about ISO C constructs that are outside of the common
subset of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.

-Wcast-qual

Warn whenever a pointer is cast so as to remove a type
qualifier from the target type. For example, warn if a "const char
*" is cast to an ordinary "char *".

-Wcast-align

Warn whenever a pointer is cast such that the required
alignment of the target is increased. For example, warn if a "char
*" is cast to an "int *" on machines where integers can
only be accessed at two- or four-byte boundaries.

-Wwrite-strings

When compiling C, give string constants the type
"const char[ length]" so that copying the address of one
into a non-"const" "char *" pointer will get a
warning; when compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning, by default, is
enabled for C++ programs. These warnings will help you find at compile
time code that can try to write into a string constant, but only if you
have been very careful about using "const" in declarations and
prototypes. Otherwise, it will just be a nuisance; this is why we did not
make -Wall request these warnings.

-Wconversion

Warn if a prototype causes a type conversion that is
different from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating and vice
versa, and conversions changing the width or signedness of a fixed point
argument except when the same as the default promotion.

Also, warn if a negative integer constant expression is implicitly converted
to an unsigned type. For example, warn about the assignment "x =
-1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".

-Wsign-compare

Warn when a comparison between signed and unsigned values
could produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the other
warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.

-Waddress

Warn about suspicious uses of memory addresses. These
include using the address of a function in a conditional expression, such
as "void func(void); if (func)", and comparisons against the
memory address of a string literal, such as "if (x ==
"abc")". Such uses typically indicate a programmer error:
the address of a function always evaluates to true, so their use in a
conditional usually indicate that the programmer forgot the parentheses in
a function call; and comparisons against string literals result in
unspecified behavior and are not portable in C, so they usually indicate
that the programmer intended to use "strcmp". This warning is
enabled by -Wall.

-Waggregate-return

Warn if any functions that return structures or unions are
defined or called. (In languages where you can return an array, this also
elicits a warning.)

-Wno-attributes

Do not warn if an unexpected "__attribute__" is
used, such as unrecognized attributes, function attributes applied to
variables, etc. This will not stop errors for incorrect use of supported
attributes.

-Wstrict-prototypes (C only)

Warn if a function is declared or defined without
specifying the argument types. (An old-style function definition is
permitted without a warning if preceded by a declaration which specifies
the argument types.)

-Wold-style-definition (C only)

Warn if an old-style function definition is used. A warning
is given even if there is a previous prototype.

-Wmissing-prototypes (C only)

Warn if a global function is defined without a previous
prototype declaration. This warning is issued even if the definition
itself provides a prototype. The aim is to detect global functions that
fail to be declared in header files.

-Wmissing-declarations (C only)

Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a prototype. Use
this option to detect global functions that are not declared in header
files.

-Wmissing-field-initializers

Warn if a structure's initializer has some fields missing.
For example, the following code would cause such a warning, because
"x.h" is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

This option does not warn about designated initializers, so the following
modification would not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-missing-field-initializers.

-Wmissing-noreturn

Warn about functions which might be candidates for
attribute "noreturn". Note these are only possible candidates,
not absolute ones. Care should be taken to manually verify functions
actually do not ever return before adding the "noreturn"
attribute, otherwise subtle code generation bugs could be introduced. You
will not get a warning for "main" in hosted C environments.

-Wmissing-format-attribute

Warn about function pointers which might be candidates for
"format" attributes. Note these are only possible candidates,
not absolute ones. GCC will guess that function pointers with
"format" attributes that are used in assignment, initialization,
parameter passing or return statements should have a corresponding
"format" attribute in the resulting type. I.e. the left-hand
side of the assignment or initialization, the type of the parameter
variable, or the return type of the containing function respectively
should also have a "format" attribute to avoid the warning.

GCC will also warn about function definitions which might be candidates for
"format" attributes. Again, these are only possible candidates.
GCC will guess that "format" attributes might be appropriate for
any function that calls a function like "vprintf" or
"vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not
be detected.

-Wno-multichar

Do not warn if a multicharacter constant ('FOOF') is
used. Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.

-Wnormalized=<none⎪id⎪nfc⎪nfkc>

In ISO C and ISO C++, two identifiers are different if they
are different sequences of characters. However, sometimes when characters
outside the basic ASCII character set are used, you can have two different
character sequences that look the same. To avoid confusion, the ISO 10646
standard sets out some normalization rules which when applied
ensure that two sequences that look the same are turned into the same
sequence. GCC can warn you if you are using identifiers which have not
been normalized; this option controls that warning.

There are four levels of warning that GCC supports. The default is
-Wnormalized=nfc, which warns about any identifier which is not in
the ISO 10646 "C" normalized form, NFC. NFC is the
recommended form for most uses.

Unfortunately, there are some characters which ISO C and ISO C++ allow in
identifiers that when turned into NFC aren't allowable as identifiers.
That is, there's no way to use these symbols in portable ISO C or C++ and
have all your identifiers in NFC. -Wnormalized=id suppresses the
warning for these characters. It is hoped that future versions of the
standards involved will correct this, which is why this option is not the
default.

You can switch the warning off for all characters by writing
-Wnormalized=none. You would only want to do this if you were using
some other normalization scheme (like "D"), because otherwise
you can easily create bugs that are literally impossible to see.

Some characters in ISO 10646 have distinct meanings but look identical in
some fonts or display methodologies, especially once formatting has been
applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL
LETTER N", will display just like a regular "n" which has
been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as well,
and GCC will warn if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about
every identifier that contains the letter O because it might be confused
with the digit 0, and so is not the default, but may be useful as a local
coding convention if the programming environment is unable to be fixed to
display these characters distinctly.

-Wno-deprecated-declarations

Do not warn about uses of functions, variables, and types
marked as deprecated by using the "deprecated" attribute.

-Wno-overflow

Do not warn about compile-time overflow in constant
expressions.

-Woverride-init

Warn if an initialized field without side effects is
overridden when using designated initializers.

This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra-Wno-override-init.

-Wpacked

Warn if a structure is given the packed attribute, but the
packed attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For instance, in
this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the
packed attribute:

Warn if padding is included in a structure, either to align
an element of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the structure
to reduce the padding and so make the structure smaller.

-Wredundant-decls

Warn if anything is declared more than once in the same
scope, even in cases where multiple declaration is valid and changes
nothing.

-Wnested-externs (C only)

Warn if an "extern" declaration is encountered
within a function.

-Wunreachable-code

Warn if the compiler detects that code will never be
executed.

This option is intended to warn when the compiler detects that at least a
whole line of source code will never be executed, because some condition
is never satisfied or because it is after a procedure that never returns.

It is possible for this option to produce a warning even though there are
circumstances under which part of the affected line can be executed, so
care should be taken when removing apparently-unreachable code.

For instance, when a function is inlined, a warning may mean that the line
is unreachable in only one inlined copy of the function.

This option is not made part of -Wall because in a debugging version
of a program there is often substantial code which checks correct
functioning of the program and is, hopefully, unreachable because the
program does work. Another common use of unreachable code is to provide
behavior which is selectable at compile-time.

-Winline

Warn if a function can not be inlined and it was declared
as inline. Even with this option, the compiler will not warn about
failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to
inline a function. For example, the compiler takes into account the size
of the function being inlined and the amount of inlining that has already
been done in the current function. Therefore, seemingly insignificant
changes in the source program can cause the warnings produced by
-Winline to appear or disappear.

-Wno-invalid-offsetof (C++ only)

Suppress warnings from applying the offsetof macro
to a non-POD type. According to the 1998 ISO C++ standard, applying
offsetof to a non-POD type is undefined. In existing C++
implementations, however, offsetof typically gives meaningful
results even when applied to certain kinds of non-POD types. (Such as a
simple struct that fails to be a POD type only by virtue of having
a constructor.) This flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to ignore the warning
about it.

The restrictions on offsetof may be relaxed in a future version of
the C++ standard.

-Wno-int-to-pointer-cast (C only)

Suppress warnings from casts to pointer type of an integer
of a different size.

-Wno-pointer-to-int-cast (C only)

Suppress warnings from casts from a pointer to an integer
type of a different size.

-Winvalid-pch

Warn if a precompiled header is found in the search path
but can't be used.

-Wlong-long

Warn if long long type is used. This is default. To
inhibit the warning messages, use -Wno-long-long. Flags
-Wlong-long and -Wno-long-long are taken into account only
when -pedantic flag is used.

-Wvariadic-macros

Warn if variadic macros are used in pedantic ISO C90 mode,
or the GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use
-Wno-variadic-macros.

-Wvolatile-register-var

Warn if a register variable is declared volatile. The
volatile modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables.

-Wdisabled-optimization

Warn if a requested optimization pass is disabled. This
warning does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers were unable to handle the
code effectively. Often, the problem is that your code is too big or too
complex; GCC will refuse to optimize programs when the optimization itself
is likely to take inordinate amounts of time.

-Wpointer-sign

Warn for pointer argument passing or assignment with
different signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -pedantic, which can be
disabled with -Wno-pointer-sign.

-Werror

Make all warnings into errors.

-Werror=

Make the specified warning into an errors. The specifier
for a warning is appended, for example -Werror=switch turns the
warnings controlled by -Wswitch into errors. This switch takes a
negative form, to be used to negate -Werror for specific warnings,
for example -Wno-error=switch makes -Wswitch warnings not be
errors, even when -Werror is in effect. You can use the
-fdiagnostics-show-option option to have each controllable warning
amended with the option which controls it, to determine what to use with
this option.

Note that specifying -Werror=foo automatically implies
-Wfoo. However, -Wno-error=foo does not imply
anything.

-Wstack-protector

This option is only active when -fstack-protector is
active. It warns about functions that will not be protected against stack
smashing.

-Woverlength-strings

Warn about string constants which are longer than the
"minimum maximum" length specified in the C standard. Modern
compilers generally allow string constants which are much longer than the
standard's minimum limit, but very portable programs should avoid using
longer strings.

The limit applies after string constant concatenation, and does not
count the trailing NUL. In C89, the limit was 509 characters; in C99, it
was raised to 4095. C++98 does not specify a normative minimum maximum, so
we do not diagnose overlength strings in C++.

This option is implied by -pedantic, and can be disabled with
-Wno-overlength-strings.

Options for Debugging Your Program or GCC

GCC has various special options that are used for debugging either your program
or GCC:

-g

Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this
debugging information.

On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information makes
debugging work better in GDB but will probably make other debuggers crash
or refuse to read the program. If you want to control for certain whether
to generate the extra information, use -gstabs+, -gstabs,
-gxcoff+, -gxcoff, or -gvms (see below).

GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally produce surprising results: some variables
you declared may not exist at all; flow of control may briefly move where
you did not expect it; some statements may not be executed because they
compute constant results or their values were already at hand; some
statements may execute in different places because they were moved out of
loops.

Nevertheless it proves possible to debug optimized output. This makes it
reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GCC is generated with the capability
for more than one debugging format.

-ggdb

Produce debugging information for use by GDB. This means to
use the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions if at
all possible.

-gstabs

Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX on most
BSD systems. On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output which is not understood by DBX or SDB. On
System V Release 4 systems this option requires the GNU assembler.

-feliminate-unused-debug-symbols

Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.

-femit-class-debug-always

Instead of emitting debugging information for a C++ class
in only one object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle the
way GCC normally emits debugging information for classes because using
this option will increase the size of debugging information by as much as
a factor of two.

-gstabs+

Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program.

-gcoff

Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V systems prior
to System V Release 4.

-gxcoff

Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM RS/6000
systems.

-gxcoff+

Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.

-gdwarf-2

Produce debugging information in DWARF version 2 format (if
that is supported). This is the format used by DBX on IRIX 6. With this
option, GCC uses features of DWARF version 3 when they are useful; version
3 is upward compatible with version 2, but may still cause problems for
older debuggers.

-gvms

Produce debugging information in VMS debug format (if that
is supported). This is the format used by DEBUG on VMS systems.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gvmslevel

Request debugging information and also use level to
specify how much information. The default level is 2.

Level 1 produces minimal information, enough for making backtraces in parts
of the program that you don't plan to debug. This includes descriptions of
functions and external variables, but no information about local variables
and no line numbers.

Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when you
use -g3.

-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very different from
version 2), and it would have been too confusing. That debug format is
long obsolete, but the option cannot be changed now. Instead use an
additional -glevel option to change the debug level for
DWARF2.

-feliminate-dwarf2-dups

Compress DWARF2 debugging information by eliminating
duplicated information about each symbol. This option only makes sense
when generating DWARF2 debugging information with -gdwarf-2.

-p

Generate extra code to write profile information suitable
for the analysis program prof. You must use this option when
compiling the source files you want data about, and you must also use it
when linking.

-pg

Generate extra code to write profile information suitable
for the analysis program gprof. You must use this option when
compiling the source files you want data about, and you must also use it
when linking.

-Q

Makes the compiler print out each function name as it is
compiled, and print some statistics about each pass when it finishes.

-ftime-report

Makes the compiler print some statistics about the time
consumed by each pass when it finishes.

-fmem-report

Makes the compiler print some statistics about permanent
memory allocation when it finishes.

-fprofile-arcs

Add code so that program flow arcs are instrumented.
During execution the program records how many times each branch and call
is executed and how many times it is taken or returns. When the compiled
program exits it saves this data to a file called
auxname.gcda for each source file. The data may be
used for profile-directed optimizations ( -fbranch-probabilities),
or for test coverage analysis ( -ftest-coverage). Each object
file's auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is
the basename of the source file. In both cases any suffix is removed (e.g.
foo.gcda for input file dir/foo.c, or dir/foo.gcda
for output file specified as -o dir/foo.o).

--coverage

This option is used to compile and link code instrumented
for coverage analysis. The option is a synonym for -fprofile-arcs-ftest-coverage (when compiling) and -lgcov (when linking).
See the documentation for those options for more details.

*

Compile the source files with -fprofile-arcs plus
optimization and code generation options. For test coverage analysis, use
the additional -ftest-coverage option. You do not need to profile
every source file in a program.

*

Link your object files with -lgcov or
-fprofile-arcs (the latter implies the former).

*

Run the program on a representative workload to generate
the arc profile information. This may be repeated any number of times. You
can run concurrent instances of your program, and provided that the file
system supports locking, the data files will be correctly updated. Also
"fork" calls are detected and correctly handled (double counting
will not happen).

*

For profile-directed optimizations, compile the source
files again with the same optimization and code generation options plus
-fbranch-probabilities.

*

For test coverage analysis, use gcov to produce
human readable information from the .gcno and .gcda files.
Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a
program flow graph, then finds a spanning tree for the graph. Only arcs that
are not on the spanning tree have to be instrumented: the compiler adds code
to count the number of times that these arcs are executed. When an arc is the
only exit or only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to hold the
instrumentation code.

-ftest-coverage

Produce a notes file that the gcov code-coverage
utility can use to show program coverage. Each source file's note file is
called auxname.gcno. Refer to the
-fprofile-arcs option above for a description of auxname and
instructions on how to generate test coverage data. Coverage data will
match the source files more closely, if you do not optimize.

-dletters

-fdump-rtl-pass

Says to make debugging dumps during compilation at times
specified by letters. This is used for debugging the RTL-based
passes of the compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname. dumpname
is generated from the name of the output file, if explicitly specified and
it is not an executable, otherwise it is the basename of the source file.

Most debug dumps can be enabled either passing a letter to the -d
option, or with a long -fdump-rtl switch; here are the possible
letters for use in letters and pass, and their
meanings:

When doing debugging dumps (see -d option above),
suppress instruction numbers, line number note and address output. This
makes it more feasible to use diff on debugging dumps for compiler
invocations with different options, in particular with and without
-g.

-fdump-translation-unit (C++ only)

-fdump-translation-unit-options(C++
only)

Dump a representation of the tree structure for the entire
translation unit to a file. The file name is made by appending .tu
to the source file name. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.

-fdump-class-hierarchy (C++ only)

-fdump-class-hierarchy-options(C++
only)

Dump a representation of each class's hierarchy and virtual
function table layout to a file. The file name is made by appending
.class to the source file name. If the -options form
is used, options controls the details of the dump as described for
the -fdump-tree options.

-fdump-ipa-switch

Control the dumping at various stages of inter-procedural
analysis language tree to a file. The file name is generated by appending
a switch specific suffix to the source file name. The following dumps are
possible:

all

Enables all inter-procedural analysis dumps; currently the
only produced dump is the cgraph dump.

Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is generated by
appending a switch specific suffix to the source file name. If the
-options form is used, options is a list of -
separated options that control the details of the dump. Not all options
are applicable to all dumps, those which are not meaningful will be
ignored. The following options are available

address

Print the address of each node. Usually this is not
meaningful as it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug environment.

slim

Inhibit dumping of members of a scope or body of a function
merely because that scope has been reached. Only dump such items when they
are directly reachable by some other path. When dumping pretty-printed
trees, this option inhibits dumping the bodies of control structures.

raw

Print a raw representation of the tree. By default, trees
are pretty-printed into a C-like representation.

details

Enable more detailed dumps (not honored by every dump
option).

stats

Enable dumping various statistics about the pass (not
honored by every dump option).

blocks

Enable showing basic block boundaries (disabled in raw
dumps).

vops

Enable showing virtual operands for every statement.

lineno

Enable showing line numbers for statements.

uid

Enable showing the unique ID ("DECL_UID") for
each variable.

all

Turn on all options, except raw, slim and
lineno.

The following tree dumps are possible:

original

Dump before any tree based optimization, to
file.original.

optimized

Dump after all tree based optimization, to
file.optimized.

inlined

Dump after function inlining, to
file.inlined.

gimple

Dump each function before and after the gimplification pass
to a file. The file name is made by appending .gimple to the source
file name.

cfg

Dump the control flow graph of each function to a file. The
file name is made by appending .cfg to the source file name.

vcg

Dump the control flow graph of each function to a file in
VCG format. The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one function, the
generated file cannot be used directly by VCG. You will need to cut and
paste each function's graph into its own separate file first.

ch

Dump each function after copying loop headers. The file
name is made by appending .ch to the source file name.

ssa

Dump SSA related information to a file. The file name is
made by appending .ssa to the source file name.

salias

Dump structure aliasing variable information to a file.
This file name is made by appending .salias to the source file
name.

alias

Dump aliasing information for each function. The file name
is made by appending .alias to the source file name.

ccp

Dump each function after CCP. The file name is made by
appending .ccp to the source file name.

storeccp

Dump each function after STORE-CCP. The file name is made
by appending .storeccp to the source file name.

pre

Dump trees after partial redundancy elimination. The file
name is made by appending .pre to the source file name.

fre

Dump trees after full redundancy elimination. The file name
is made by appending .fre to the source file name.

copyprop

Dump trees after copy propagation. The file name is made by
appending .copyprop to the source file name.

store_copyprop

Dump trees after store copy-propagation. The file name is
made by appending .store_copyprop to the source file name.

dce

Dump each function after dead code elimination. The file
name is made by appending .dce to the source file name.

mudflap

Dump each function after adding mudflap instrumentation.
The file name is made by appending .mudflap to the source file
name.

sra

Dump each function after performing scalar replacement of
aggregates. The file name is made by appending .sra to the source
file name.

sink

Dump each function after performing code sinking. The file
name is made by appending .sink to the source file name.

dom

Dump each function after applying dominator tree
optimizations. The file name is made by appending .dom to the
source file name.

dse

Dump each function after applying dead store elimination.
The file name is made by appending .dse to the source file
name.

phiopt

Dump each function after optimizing PHI nodes into
straightline code. The file name is made by appending .phiopt to
the source file name.

forwprop

Dump each function after forward propagating single use
variables. The file name is made by appending .forwprop to the
source file name.

copyrename

Dump each function after applying the copy rename
optimization. The file name is made by appending .copyrename to the
source file name.

nrv

Dump each function after applying the named return value
optimization on generic trees. The file name is made by appending
.nrv to the source file name.

vect

Dump each function after applying vectorization of loops.
The file name is made by appending .vect to the source file
name.

vrp

Dump each function after Value Range Propagation (VRP). The
file name is made by appending .vrp to the source file name.

all

Enable all the available tree dumps with the flags provided
in this option.

-ftree-vectorizer-verbose=n

This option controls the amount of debugging output the
vectorizer prints. This information is written to standard error, unless
-fdump-tree-all or -fdump-tree-vect is specified, in which
case it is output to the usual dump listing file, .vect. For
n=0 no diagnostic information is reported. If n=1 the
vectorizer reports each loop that got vectorized, and the total number of
loops that got vectorized. If n=2 the vectorizer also reports
non-vectorized loops that passed the first analysis phase
(vect_analyze_loop_form) - i.e. countable, inner-most, single-bb,
single-entry/exit loops. This is the same verbosity level that
-fdump-tree-vect-stats uses. Higher verbosity levels mean either
more information dumped for each reported loop, or same amount of
information reported for more loops: If n=3, alignment related
information is added to the reports. If n=4, data-references
related information (e.g. memory dependences, memory access-patterns) is
added to the reports. If n=5, the vectorizer reports also
non-vectorized inner-most loops that did not pass the first analysis phase
(i.e. may not be countable, or may have complicated control-flow). If
n=6, the vectorizer reports also non-vectorized nested loops. For
n=7, all the information the vectorizer generates during its
analysis and transformation is reported. This is the same verbosity level
that -fdump-tree-vect-details uses.

-frandom-seed=string

This option provides a seed that GCC uses when it would
otherwise use random numbers. It is used to generate certain symbol names
that have to be different in every compiled file. It is also used to place
unique stamps in coverage data files and the object files that produce
them. You can use the -frandom-seed option to produce reproducibly
identical object files.

The string should be different for every file you compile.

-fsched-verbose=n

On targets that use instruction scheduling, this option
controls the amount of debugging output the scheduler prints. This
information is written to standard error, unless -dS or -dR
is specified, in which case it is output to the usual dump listing file,
.sched or .sched2 respectively. However for n greater
than nine, the output is always printed to standard error.

For n greater than zero, -fsched-verbose outputs the same
information as -dRS. For n greater than one, it also output
basic block probabilities, detailed ready list information and unit/insn
info. For n greater than two, it includes RTL at abort point,
control-flow and regions info. And for n over four,
-fsched-verbose also includes dependence info.

-save-temps

Store the usual "temporary" intermediate files
permanently; place them in the current directory and name them based on
the source file. Thus, compiling foo.c with -c -save-temps
would produce files foo.i and foo.s, as well as
foo.o. This creates a preprocessed foo.i output file even
though the compiler now normally uses an integrated preprocessor.

When used in combination with the -x command line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the source
file before using -save-temps.

-time

Report the CPU time taken by each subprocess in the
compilation sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done). The output looks like
this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.

-fvar-tracking

Run variable tracking pass. It computes where variables are
stored at each position in code. Better debugging information is then
generated (if the debugging information format supports this information).

Print the full absolute name of the library file
library that would be used when linking---and don't do anything
else. With this option, GCC does not compile or link anything; it just
prints the file name.

-print-multi-directory

Print the directory name corresponding to the multilib
selected by any other switches present in the command line. This directory
is supposed to exist in GCC_EXEC_PREFIX.

-print-multi-lib

Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from the
switches by ;, and each switch starts with an @} instead of
the@samp{-, without spaces between multiple
switches. This is supposed to ease shell-processing.

-print-prog-name=program

Like -print-file-name, but searches for a program
such as cpp.

-print-libgcc-file-name

Same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but
you do want to link with libgcc.a. You can do

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

-print-search-dirs

Print the name of the configured installation directory and
a list of program and library directories gcc will search---and
don't do anything else.

This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To resolve this
you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed them. Don't
forget the trailing /.

Print the compiler's built-in specs---and don't do anything
else. (This is used when GCC itself is being built.)

-feliminate-unused-debug-types

Normally, when producing DWARF2 output, GCC will emit
debugging information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that compilation
unit. Sometimes this is useful, such as if, in the debugger, you want to
cast a value to a type that is not actually used in your program (but is
declared). More often, however, this results in a significant amount of
wasted space. With this option, GCC will avoid producing debug symbol
output for types that are nowhere used in the source file being compiled.

Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results. Statements are
independent: if you stop the program with a breakpoint between statements, you
can then assign a new value to any variable or change the program counter to
any other statement in the function and get exactly the results you would
expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and possibly
the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program.
Optimization levels -O and above, in particular, enable
unit-at-a-time mode, which allows the compiler to consider information
gained from later functions in the file when compiling a function. Compiling
multiple files at once to a single output file in unit-at-a-time mode
allows the compiler to use information gained from all of the files when
compiling each of them.

Not all optimizations are controlled directly by a flag. Only optimizations that
have a flag are listed.

-O

-O1

Optimize. Optimizing compilation takes somewhat more time,
and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of compilation
time.

-O turns on the following optimization flags: -fdefer-pop-fdelayed-branch-fguess-branch-probability-fcprop-registers-fif-conversion-fif-conversion2-ftree-ccp-ftree-dce-ftree-dominator-opts-ftree-dse-ftree-ter-ftree-lrs-ftree-sra-ftree-copyrename-ftree-fre-ftree-ch-funit-at-a-time-fmerge-constants

-O also turns on -fomit-frame-pointer on machines where doing
so does not interfere with debugging.

-O2

Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. The compiler
does not perform loop unrolling or function inlining when you specify
-O2. As compared to -O, this option increases both
compilation time and the performance of the generated code.

-O2 turns on all optimization flags specified by -O. It also
turns on the following optimization flags: -fthread-jumps-fcrossjumping-foptimize-sibling-calls-fcse-follow-jumps -fcse-skip-blocks-fgcse -fgcse-lm-fexpensive-optimizations-frerun-cse-after-loop-fcaller-saves-fpeephole2-fschedule-insns
-fschedule-insns2-fsched-interblock -fsched-spec-fregmove-fstrict-aliasing -fstrict-overflow-fdelete-null-pointer-checks-freorder-blocks
-freorder-functions-falign-functions -falign-jumps-falign-loops -falign-labels-ftree-vrp-ftree-pre

Please note the warning under -fgcse about invoking -O2 on
programs that use computed gotos.

-O2 doesn't turn on -ftree-vrp for the Ada compiler. This
option must be explicitly specified on the command line to be enabled for
the Ada compiler.

-O3

Optimize yet more. -O3 turns on all optimizations
specified by -O2 and also turns on the -finline-functions,
-funswitch-loops and -fgcse-after-reload options.

-O0

Do not optimize. This is the default.

-Os

Optimize for size. -Os enables all -O2
optimizations that do not typically increase code size. It also performs
further optimizations designed to reduce code size.

If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.

Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will use. You can figure out the other form by
either removing no- or adding it.

The following options control specific optimizations. They are either activated
by -O options or are related to ones that are. You can use the
following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.

-fno-default-inline

Do not make member functions inline by default merely
because they are defined inside the class scope (C++ only). Otherwise,
when you specify -O, member functions defined inside class scope
are compiled inline by default; i.e., you don't need to add inline
in front of the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as
that function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on the
stack for several function calls and pops them all at once.

Disabled at levels -O, -O2, -O3, -Os.

-fforce-mem

Force memory operands to be copied into registers before
doing arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not common
subexpressions, instruction combination should eliminate the separate
register-load. This option is now a nop and will be removed in 4.3.

-fforce-addr

Force memory address constants to be copied into registers
before doing arithmetic on them.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions
that don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible onsome
machines.

On some machines, such as the VAX, this flag has no effect, because the
standard calling sequence automatically handles the frame pointer and
nothing is saved by pretending it doesn't exist. The machine-description
macro "FRAME_POINTER_REQUIRED" controls whether a target machine
supports this flag.

Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-fno-inline

Don't pay attention to the "inline" keyword.
Normally this option is used to keep the compiler from expanding any
functions inline. Note that if you are not optimizing, no functions can be
expanded inline.

-finline-functions

Integrate all simple functions into their callers. The
compiler heuristically decides which functions are simple enough to be
worth integrating in this way.

If all calls to a given function are integrated, and the function is
declared "static", then the function is normally not output as
assembler code in its own right.

Enabled at level -O3.

-finline-functions-called-once

Consider all "static" functions called once for
inlining into their caller even if they are not marked "inline".
If a call to a given function is integrated, then the function is not
output as assembler code in its own right.

Enabled if -funit-at-a-time is enabled.

-fearly-inlining

Inline functions marked by "always_inline" and
functions whose body seems smaller than the function call overhead early
before doing -fprofile-generate instrumentation and real inlining
pass. Doing so makes profiling significantly cheaper and usually inlining
faster on programs having large chains of nested wrapper functions.

Enabled by default.

-finline-limit=n

By default, GCC limits the size of functions that can be
inlined. This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword or
defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions (not
counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost of
compilation time and memory consumption. Decreasing usually makes the
compilation faster and less code will be inlined (which presumably means
slower programs). This option is particularly useful for programs that use
inlining heavily such as those based on recursive templates with C++.

Inlining is actually controlled by a number of parameters, which may be
specified individually by using --paramname=value. The -finline-limit=n option
sets some of these parameters as follows:

max-inline-insns-single

is set to I<n>/2.

max-inline-insns-auto

is set to I<n>/2.

min-inline-insns

is set to 130 or I<n>/4, whichever is smaller.

max-inline-insns-rtl

is set to I<n>.

See below for a documentation of the individual parameters controlling inlining.

Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it represent a count
of assembly instructions and as such its exact meaning might change from one
release to an another.

-fkeep-inline-functions

In C, emit "static" functions that are declared
"inline" into the object file, even if the function has been
inlined into all of its callers. This switch does not affect functions
using the "extern inline" extension in GNU C. In C++, emit any
and all inline functions into the object file.

GCC enables this option by default. If you want to force the compiler to
check if the variable was referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts
option.

This option is the default for optimized compilation if the assembler and
linker support it. Use -fno-merge-constants to inhibit this
behavior.

Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants

Attempt to merge identical constants and identical
variables.

This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating point
types. Languages like C or C++ require each non-automatic variable to have
distinct location, so using this option will result in non-conforming
behavior.

-fmodulo-sched

Perform swing modulo scheduling immediately before the
first scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.

-fno-branch-count-reg

Do not use "decrement and branch" instructions on
a count register, but instead generate a sequence of instructions that
decrement a register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.

The default is -fbranch-count-reg.

-fno-function-cse

Do not put function addresses in registers; make each
instruction that calls a constant function contain the function's address
explicitly.

This option results in less efficient code, but some strange hacks that
alter the assembler output may be confused by the optimizations performed
when this option is not used.

The default is -ffunction-cse

-fno-zero-initialized-in-bss

If the target supports a BSS section, GCC by default puts
variables that are initialized to zero into BSS. This can save space in
the resulting code.

This option turns off this behavior because some programs explicitly rely on
variables going to the data section. E.g., so that the resulting
executable can find the beginning of that section and/or make assumptions
based on that.

The default is -fzero-initialized-in-bss.

-fbounds-check

For front-ends that support it, generate additional code to
check that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran front-ends, where
this option defaults to true and false respectively.

-fmudflap -fmudflapth -fmudflapir

For front-ends that support it (C and C++), instrument all
risky pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to buffer
overflows, invalid heap use, and some other classes of C/C++ programming
errors. The instrumentation relies on a separate runtime library (
libmudflap), which will be linked into a program if
-fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for
its options.

Use -fmudflapth instead of -fmudflap to compile and to link if
your program is multi-threaded. Use -fmudflapir, in addition to
-fmudflap or -fmudflapth, if instrumentation should ignore
pointer reads. This produces less instrumentation (and therefore faster
execution) and still provides some protection against outright memory
corrupting writes, but allows erroneously read data to propagate within a
program.

-fthread-jumps

Perform optimizations where we check to see if a jump
branches to a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.

Enabled at levels -O2, -O3, -Os.

-fcse-follow-jumps

In common subexpression elimination, scan through jump
instructions when the target of the jump is not reached by any other path.
For example, when CSE encounters an "if" statement with an
"else" clause, CSE will follow the jump when the condition
tested is false.

Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks

This is similar to -fcse-follow-jumps, but causes
CSE to follow jumps which conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the body of
the "if".

Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop
optimizations has been performed.

Enabled at levels -O2, -O3, -Os.

-fgcse

Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC
extension, you may get better runtime performance if you disable the
global common subexpression elimination pass by adding -fno-gcse to
the command line.

Enabled at levels -O2, -O3, -Os.

-fgcse-lm

When -fgcse-lm is enabled, global common
subexpression elimination will attempt to move loads which are only killed
by stores into themselves. This allows a loop containing a load/store
sequence to be changed to a load outside the loop, and a copy/store within
the loop.

Enabled by default when gcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is
run after global common subexpression elimination. This pass will attempt
to move stores out of loops. When used in conjunction with
-fgcse-lm, loops containing a load/store sequence can be changed to
a load before the loop and a store after the loop.

Not enabled at any optimization level.

-fgcse-las

When -fgcse-las is enabled, the global common
subexpression elimination pass eliminates redundant loads that come after
stores to the same memory location (both partial and full redundancies).

Not enabled at any optimization level.

-fgcse-after-reload

When -fgcse-after-reload is enabled, a redundant
load elimination pass is performed after reload. The purpose of this pass
is to cleanup redundant spilling.

-funsafe-loop-optimizations

If given, the loop optimizer will assume that loop indices
do not overflow, and that the loops with nontrivial exit condition are not
infinite. This enables a wider range of loop optimizations even if the
loop optimizer itself cannot prove that these assumptions are valid. Using
-Wunsafe-loop-optimizations, the compiler will warn you if it finds
this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation
unifies equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.

Enabled at levels -O2, -O3, -Os.

-fif-conversion

Attempt to transform conditional jumps into branch-less
equivalents. This include use of conditional moves, min, max, set flags
and abs instructions, and some tricks doable by standard arithmetics. The
use of conditional execution on chips where it is available is controlled
by "if-conversion2".

Use global dataflow analysis to identify and eliminate
useless checks for null pointers. The compiler assumes that dereferencing
a null pointer would have halted the program. If a pointer is checked
after it has already been dereferenced, it cannot be null.

In some environments, this assumption is not true, and programs can safely
dereference null pointers. Use -fno-delete-null-pointer-checks to
disable this optimization for programs which depend on that behavior.

Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively
expensive.

Enabled at levels -O2, -O3, -Os.

-foptimize-register-move

-fregmove

Attempt to reassign register numbers in move instructions
and as operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines with
two-operand instructions.

Note -fregmove and -foptimize-register-move are the same
optimization.

Enabled at levels -O2, -O3, -Os.

-fdelayed-branch

If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed branch
instructions.

Enabled at levels -O, -O2, -O3, -Os.

-fschedule-insns

If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point or memory
load instructions by allowing other instructions to be issued until the
result of the load or floating point instruction is required.

Enabled at levels -O2, -O3, -Os.

-fschedule-insns2

Similar to -fschedule-insns, but requests an
additional pass of instruction scheduling after register allocation has
been done. This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more than one
cycle.

Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock

Don't schedule instructions across basic blocks. This is
normally enabled by default when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec

Don't allow speculative motion of non-load instructions.
This is normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or
higher.

-fsched-spec-load

Allow speculative motion of some load instructions. This
only makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous

Allow speculative motion of more load instructions. This
only makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.

-fsched-stalled-insns=n

Define how many insns (if any) can be moved prematurely
from the queue of stalled insns into the ready list, during the second
scheduling pass.

-fsched-stalled-insns-dep=n

Define how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is candidate for premature removal from
the queue of stalled insns. Has an effect only during the second
scheduling pass, and only if -fsched-stalled-insns is used and its
value is not zero.

-fsched2-use-superblocks

When scheduling after register allocation, do use
superblock scheduling algorithm. Superblock scheduling allows motion
across basic block boundaries resulting on faster schedules. This option
is experimental, as not all machine descriptions used by GCC model the CPU
closely enough to avoid unreliable results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.

-fsched2-use-traces

Use -fsched2-use-superblocks algorithm when
scheduling after register allocation and additionally perform code
duplication in order to increase the size of superblocks using tracer
pass. See -ftracer for details on trace formation.

This mode should produce faster but significantly longer programs. Also
without -fbranch-probabilities the traces constructed may not match
the reality and hurt the performance. This only makes sense when
scheduling after register allocation, i.e. with -fschedule-insns2
or at -O2 or higher.

The modulo scheduling comes before the traditional
scheduling, if a loop was modulo scheduled we may want to prevent the
later scheduling passes from changing its schedule, we use this option to
control that.

-fcaller-saves

Enable values to be allocated in registers that will be
clobbered by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done only when
it seems to result in better code than would otherwise be produced.

This option is always enabled by default on certain machines, usually those
which have no call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os.

-ftree-pre

Perform Partial Redundancy Elimination (PRE) on trees. This
flag is enabled by default at -O2 and -O3.

-ftree-fre

Perform Full Redundancy Elimination (FRE) on trees. The
difference between FRE and PRE is that FRE only considers expressions that
are computed on all paths leading to the redundant computation. This
analysis faster than PRE, though it exposes fewer redundancies. This flag
is enabled by default at -O and higher.

-ftree-copy-prop

Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at -O
and higher.

Perform structural alias analysis on trees. This flag is
enabled by default at -O and higher.

-fipa-pta

Perform interprocedural pointer analysis.

-ftree-sink

Perform forward store motion on trees. This flag is enabled
by default at -O and higher.

-ftree-ccp

Perform sparse conditional constant propagation (CCP) on
trees. This pass only operates on local scalar variables and is enabled by
default at -O and higher.

-ftree-store-ccp

Perform sparse conditional constant propagation (CCP) on
trees. This pass operates on both local scalar variables and memory stores
and loads (global variables, structures, arrays, etc). This flag is
enabled by default at -O2 and higher.

-ftree-dce

Perform dead code elimination (DCE) on trees. This flag is
enabled by default at -O and higher.

-ftree-dominator-opts

Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also performs
jump threading (to reduce jumps to jumps). This flag is enabled by default
at -O and higher.

-ftree-ch

Perform loop header copying on trees. This is beneficial
since it increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at -O and higher.
It is not enabled for -Os, since it usually increases code
size.

-ftree-loop-optimize

Perform loop optimizations on trees. This flag is enabled
by default at -O and higher.

-ftree-loop-linear

Perform linear loop transformations on tree. This flag can
improve cache performance and allow further loop optimizations to take
place.

-ftree-loop-im

Perform loop invariant motion on trees. This pass moves
only invariants that would be hard to handle at RTL level (function calls,
operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial invariantness
analysis in loop unswitching. The pass also includes store motion.

-ftree-loop-ivcanon

Create a canonical counter for number of iterations in the
loop for that determining number of iterations requires complicated
analysis. Later optimizations then may determine the number easily. Useful
especially in connection with unrolling.

Perform scalar replacement of aggregates. This pass
replaces structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by default at
-O and higher.

-ftree-copyrename

Perform copy renaming on trees. This pass attempts to
rename compiler temporaries to other variables at copy locations, usually
resulting in variable names which more closely resemble the original
variables. This flag is enabled by default at -O and higher.

-ftree-ter

Perform temporary expression replacement during the
SSA->normal phase. Single use/single def temporaries are replaced at
their use location with their defining expression. This results in
non-GIMPLE code, but gives the expanders much more complex trees to work
on resulting in better RTL generation. This is enabled by default at
-O and higher.

-ftree-lrs

Perform live range splitting during the SSA->normal
phase. Distinct live ranges of a variable are split into unique variables,
allowing for better optimization later. This is enabled by default at
-O and higher.

-ftree-vectorize

Perform loop vectorization on trees.

-ftree-vect-loop-version

Perform loop versioning when doing loop vectorization on
trees. When a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time then vectorized
and non-vectorized versions of the loop are generated along with runtime
checks for alignment or dependence to control which version is executed.
This option is enabled by default except at level -Os where it is
disabled.

-ftree-vrp

Perform Value Range Propagation on trees. This is similar
to the constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary range
checks like array bound checks and null pointer checks. This is enabled by
default at -O2 and higher. Null pointer check elimination is only
done if -fdelete-null-pointer-checks is enabled.

-ftracer

Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing other
optimizations to do better job.

-funroll-loops

Unroll loops whose number of iterations can be determined
at compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop. This option makes code larger, and may or
may not make it run faster.

-funroll-all-loops

Unroll all loops, even if their number of iterations is
uncertain when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops,

-fsplit-ivs-in-unroller

Enables expressing of values of induction variables in
later iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving efficiency
of the scheduling passes.

Combination of -fweb and CSE is often sufficient to obtain the same
effect. However in cases the loop body is more complicated than a single
basic block, this is not reliable. It also does not work at all on some of
the architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller

With this option, the compiler will create multiple copies
of some local variables when unrolling a loop which can result in superior
code.

-fprefetch-loop-arrays

If supported by the target machine, generate instructions
to prefetch memory to improve the performance of loops that access large
arrays.

This option may generate better or worse code; results are highly dependent
on the structure of loops within the source code.

Disabled at level -Os.

-fno-peephole

-fno-peephole2

Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in
how they are implemented in the compiler; some targets use one, some use
the other, a few use both.

GCC will use heuristics to guess branch probabilities if they are not
provided by profiling feedback ( -fprofile-arcs). These heuristics
are based on the control flow graph. If some branch probabilities are
specified by __builtin_expect, then the heuristics will be used to
guess branch probabilities for the rest of the control flow graph, taking
the __builtin_expect info into account. The interactions between
the heuristics and __builtin_expect can be complex, and in some
cases, it may be useful to disable the heuristics so that the effects of
__builtin_expect are easier to understand.

The default is -fguess-branch-probability at levels -O,
-O2, -O3, -Os.

-freorder-blocks

Reorder basic blocks in the compiled function in order to
reduce number of taken branches and improve code locality.

Enabled at levels -O2, -O3.

-freorder-blocks-and-partition

In addition to reordering basic blocks in the compiled
function, in order to reduce number of taken branches, partitions hot and
cold basic blocks into separate sections of the assembly and .o files, to
improve paging and cache locality performance.

This optimization is automatically turned off in the presence of exception
handling, for linkonce sections, for functions with a user-defined section
attribute and on any architecture that does not support named
sections.

-freorder-functions

Reorder functions in the object file in order to improve
code locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named sections and
linker must place them in a reasonable way.

Also profile feedback must be available in to make this option effective.
See -fprofile-arcs for details.

Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing

Allows the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an object
of one type is assumed never to reside at the same address as an object of
a different type, unless the types are almost the same. For example, an
"unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any
other type.

Pay special attention to code like this:

union a_union {
int i;
double d;
};

int f() {
a_union t;
t.d = 3.0;
return t.i;
}

The practice of reading from a different union member than the one most
recently written to (called "type-punning") is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory is
accessed through the union type. So, the code above will work as expected.
However, this code might not:

int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}

Every language that wishes to perform language-specific alias analysis
should define a function that computes, given an "tree" node, an
alias set for the node. Nodes in different alias sets are not allowed to
alias. For an example, see the C front-end function
"c_get_alias_set".

Enabled at levels -O2, -O3, -Os.

-fstrict-overflow

Allow the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C++) this means that
overflow when doing arithmetic with signed numbers is undefined, which
means that the compiler may assume that it will not happen. This permits
various optimizations. For example, the compiler will assume that an
expression like "i + 10 > i" will always be true for signed
"i". This assumption is only valid if signed overflow is
undefined, as the expression is false if "i + 10" overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers will overflow
must be written carefully to not actually involve overflow.

See also the -fwrapv option. Using -fwrapv means that signed
overflow is fully defined: it wraps. When -fwrapv is used, there is
no difference between -fstrict-overflow and
-fno-strict-overflow. With -fwrapv certain types of overflow
are permitted. For example, if the compiler gets an overflow when doing
arithmetic on constants, the overflowed value can still be used with
-fwrapv, but not otherwise.

The -fstrict-overflow option is enabled at levels -O2,
-O3, -Os.

-falign-functions

-falign-functions=n

Align the start of functions to the next power-of-two
greater than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte boundary,
but -falign-functions=24 would align to the next 32-byte boundary
only if this can be done by skipping 23 bytes or less.

-fno-align-functions and -falign-functions=1 are equivalent
and mean that functions will not be aligned.

Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-labels

-falign-labels=n

Align all branch targets to a power-of-two boundary,
skipping up to n bytes like -falign-functions. This option
can easily make code slower, because it must insert dummy operations for
when the branch target is reached in the usual flow of the code.

-fno-align-labels and -falign-labels=1 are equivalent and
mean that labels will not be aligned.

If -falign-loops or -falign-jumps are applicable and are
greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment.

Enabled at levels -O2, -O3.

-falign-loops

-falign-loops=n

Align loops to a power-of-two boundary, skipping up to
n bytes like -falign-functions. The hope is that the loop
will be executed many times, which will make up for any execution of the
dummy operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean
that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-jumps

-falign-jumps=n

Align branch targets to a power-of-two boundary, for branch
targets where the targets can only be reached by jumping, skipping up to
n bytes like -falign-functions. In this case, no dummy
operations need be executed.

-fno-align-jumps and -falign-jumps=1 are equivalent and mean
that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-funit-at-a-time

Parse the whole compilation unit before starting to produce
code. This allows some extra optimizations to take place but consumes more
memory (in general). There are some compatibility issues with
unit-at-a-time mode:

*

enabling unit-at-a-time mode may change the order in
which functions, variables, and top-level "asm" statements are
emitted, and will likely break code relying on some particular ordering.
The majority of such top-level "asm" statements, though, can be
replaced by "section" attributes. The
fno-toplevel-reorder option may be used to keep the ordering used
in the input file, at the cost of some optimizations.

*

unit-at-a-time mode removes unreferenced static
variables and functions. This may result in undefined references when an
"asm" statement refers directly to variables or functions that
are otherwise unused. In that case either the variable/function shall be
listed as an operand of the "asm" statement operand or, in the
case of top-level "asm" statements the attribute
"used" shall be used on the declaration.

*

Static functions now can use non-standard passing
conventions that may break "asm" statements calling functions
directly. Again, attribute "used" will prevent this
behavior.

As a temporary workaround, -fno-unit-at-a-time can be used, but this
scheme may not be supported by future releases of GCC.

Enabled at levels -O, -O2, -O3, -Os.

-fno-toplevel-reorder

Do not reorder top-level functions, variables, and
"asm" statements. Output them in the same order that they appear
in the input file. When this option is used, unreferenced static variables
will not be removed. This option is intended to support existing code
which relies on a particular ordering. For new code, it is better to use
attributes.

-fweb

Constructs webs as commonly used for register allocation
purposes and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop optimizer
and trivial dead code remover. It can, however, make debugging impossible,
since variables will no longer stay in a "home register".

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents whole
program being compiled. All public functions and variables with the
exception of "main" and those merged by attribute
"externally_visible" become static functions and in a affect
gets more aggressively optimized by interprocedural optimizers. While this
option is equivalent to proper use of "static" keyword for
programs consisting of single file, in combination with option
--combine this flag can be used to compile most of smaller scale C
programs since the functions and variables become local for the whole
combined compilation unit, not for the single source file itself.

-fno-cprop-registers

After register allocation and post-register allocation
instruction splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.

Disabled at levels -O, -O2, -O3, -Os.

-fprofile-generate

Enable options usually used for instrumenting application
to produce profile useful for later recompilation with profile feedback
based optimization. You must use -fprofile-generate both when
compiling and when linking your program.

The following options are enabled: "-fprofile-arcs",
"-fprofile-values", "-fvpt".

The following options are enabled: "-fbranch-probabilities",
"-fvpt", "-funroll-loops", "-fpeel-loops",
"-ftracer"

The following options control compiler behavior regarding floating point
arithmetic. These options trade off between speed and correctness. All must be
specifically enabled.

-ffloat-store

Do not store floating point variables in registers, and
inhibit other options that might change whether a floating point value is
taken from a register or memory.

This option prevents undesirable excess precision on machines such as the
68000 where the floating registers (of the 68881) keep more precision than
a "double" is supposed to have. Similarly for the x86
architecture. For most programs, the excess precision does only good, but
a few programs rely on the precise definition of IEEE floating point. Use
-ffloat-store for such programs, after modifying them to store all
pertinent intermediate computations into variables.

This option causes the preprocessor macro "__FAST_MATH__" to be
defined.

This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.

-fno-math-errno

Do not set ERRNO after calling math functions that are
executed with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag for
speed while maintaining IEEE arithmetic compatibility.

This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.

The default is -fmath-errno.

On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility that it
might, and -fno-math-errno is the default.

-funsafe-math-optimizations

Allow optimizations for floating-point arithmetic that (a)
assume that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries or
startup files that change the default FPU control word or other similar
optimizations.

This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.

The default is -fno-unsafe-math-optimizations.

-ffinite-math-only

Allow optimizations for floating-point arithmetic that
assume that arguments and results are not NaNs or +-Infs.

This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications.

The default is -fno-finite-math-only.

-fno-trapping-math

Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation. This option
implies -fno-signaling-nans. Setting this option may allow faster
code if one relies on "non-stop" IEEE arithmetic, for example.

This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math functions.

The default is -ftrapping-math.

-frounding-math

Disable transformations and optimizations that assume
default floating point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified for programs that
change the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating point expressions at compile-time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all
GCC optimizations that are affected by rounding mode. Future versions of
GCC may provide finer control of this setting using C99's
"FENV_ACCESS" pragma. This command line option will be used to
specify the default state for "FENV_ACCESS".

-frtl-abstract-sequences

It is a size optimization method. This option is to find
identical sequences of code, which can be turned into pseudo-procedures
and then replace all occurrences with calls to the newly created
subroutine. It is kind of an opposite of -finline-functions. This
optimization runs at RTL level.

-fsignaling-nans

Compile code assuming that IEEE signaling NaNs may generate
user-visible traps during floating-point operations. Setting this option
disables optimizations that may change the number of exceptions visible
with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all
GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant

Treat floating point constant as single precision constant
instead of implicitly converting it to double precision constant.

-fcx-limited-range

-fno-cx-limited-range

When enabled, this option states that a range reduction
step is not needed when performing complex division. The default is
-fno-cx-limited-range, but is enabled by -ffast-math.

This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to
all languages.

The following options control optimizations that may improve performance, but
are not enabled by any -O options. This section includes experimental
options that may produce broken code.

-fbranch-probabilities

After running a program compiled with
-fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the
number of times each branch was taken. When the program compiled with
-fprofile-arcs exits it saves arc execution counts to a file called
sourcename.gcda for each source file The information
in this data file is very dependent on the structure of the generated
code, so you must use the same source code and the same optimization
options for both compilations.

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on
each JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is mostly to take,
the REG_BR_PROB values are used to exactly determine which path is
taken more often.

-fprofile-values

If combined with -fprofile-arcs, it adds code so
that some data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions and adds REG_VALUE_PROFILE notes to
instructions for their later usage in optimizations.

Enabled with -fprofile-generate and -fprofile-use.

-fvpt

If combined with -fprofile-arcs, it instructs the
compiler to add a code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operation using the
knowledge about the value of the denominator.

-frename-registers

Attempt to avoid false dependencies in scheduled code by
making use of registers left over after register allocation. This
optimization will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target, however,
it can make debugging impossible, since variables will no longer stay in a
"home register".

Enabled by default with -funroll-loops.

-ftracer

Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing other
optimizations to do better job.

Enabled with -fprofile-use.

-funroll-loops

Unroll loops whose number of iterations can be determined
at compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops
with small constant number of iterations). This option makes code larger,
and may or may not make it run faster.

Enabled with -fprofile-use.

-funroll-all-loops

Unroll all loops, even if their number of iterations is
uncertain when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.

-fpeel-loops

Peels the loops for that there is enough information that
they do not roll much (from profile feedback). It also turns on complete
loop peeling (i.e. complete removal of loops with small constant number of
iterations).

Move branches with loop invariant conditions out of the
loop, with duplicates of the loop on both branches (modified according to
result of the condition).

-ffunction-sections

-fdata-sections

Place each function or data item into its own section in
the output file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name in the
output file.

Use these options on systems where the linker can perform optimizations to
improve locality of reference in the instruction space. Most systems using
the ELF object format and SPARC processors running Solaris 2 have linkers
with such optimizations. AIX may have these optimizations in the future.

Only use these options when there are significant benefits from doing so.
When you specify these options, the assembler and linker will create
larger object and executable files and will also be slower. You will not
be able to use "gprof" on all systems if you specify this option
and you may have problems with debugging if you specify both this option
and -g.

-fbranch-target-load-optimize

Perform branch target register load optimization before
prologue / epilogue threading. The use of target registers can typically
be exposed only during reload, thus hoisting loads out of loops and doing
inter-block scheduling needs a separate optimization pass.

Emit extra code to check for buffer overflows, such as
stack smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that call
alloca, and functions with buffers larger than 8 bytes. The guards are
initialized when a function is entered and then checked when the function
exits. If a guard check fails, an error message is printed and the program
exits.

-fstack-protector-all

Like -fstack-protector except that all functions are
protected.

-fstack-protector-strong

Like -fstack-protector but includes additional
functions to be protected --- those that have local array definitions, or
have references to local frame addresses.

-fsection-anchors

Try to reduce the number of symbolic address calculations
by using shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and GOT
accesses on some targets.

For example, the implementation of the following function "foo":

static int a, b, c;
int foo (void) { return a + b + c; }

would usually calculate the addresses of all three variables, but if you
compile it with -fsection-anchors, it will access the variables
from a common anchor point instead. The effect is similar to the following
pseudocode (which isn't valid C):

In some places, GCC uses various constants to control the
amount of optimization that is done. For example, GCC will not inline
functions that contain more that a certain number of instructions. You can
control some of these constants on the command-line using the
--param option.

The names of specific parameters, and the meaning of the values, are tied to
the internals of the compiler, and are subject to change without notice in
future releases.

In each case, the value is an integer. The allowable choices for
name are given in the following table:

salias-max-implicit-fields

The maximum number of fields in a variable without direct
structure accesses for which structure aliasing will consider trying to
track each field. The default is 5

salias-max-array-elements

The maximum number of elements an array can have and its
elements still be tracked individually by structure aliasing. The default
is 4

sra-max-structure-size

The maximum structure size, in bytes, at which the scalar
replacement of aggregates (SRA) optimization will perform block copies.
The default value, 0, implies that GCC will select the most appropriate
size itself.

sra-field-structure-ratio

The threshold ratio (as a percentage) between instantiated
fields and the complete structure size. We say that if the ratio of the
number of bytes in instantiated fields to the number of bytes in the
complete structure exceeds this parameter, then block copies are not used.
The default is 75.

max-crossjump-edges

The maximum number of incoming edges to consider for
crossjumping. The algorithm used by -fcrossjumping is O(N^2) in the
number of edges incoming to each block. Increasing values mean more
aggressive optimization, making the compile time increase with probably
small improvement in executable size.

min-crossjump-insns

The minimum number of instructions which must be matched at
the end of two blocks before crossjumping will be performed on them. This
value is ignored in the case where all instructions in the block being
crossjumped from are matched. The default value is 5.

max-grow-copy-bb-insns

The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a jump
instruction. The default value is 8.

max-goto-duplication-insns

The maximum number of instructions to duplicate to a block
that jumps to a computed goto. To avoid O(N^2) behavior in a number of
passes, GCC factors computed gotos early in the compilation process, and
unfactors them as late as possible. Only computed jumps at the end of a
basic blocks with no more than max-goto-duplication-insns are unfactored.
The default value is 8.

max-delay-slot-insn-search

The maximum number of instructions to consider when looking
for an instruction to fill a delay slot. If more than this arbitrary
number of instructions is searched, the time savings from filling the
delay slot will be minimal so stop searching. Increasing values mean more
aggressive optimization, making the compile time increase with probably
small improvement in executable run time.

max-delay-slot-live-search

When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with valid live
register information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compile time. This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.

max-gcse-memory

The approximate maximum amount of memory that will be
allocated in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the optimization
will not be done.

max-gcse-passes

The maximum number of passes of GCSE to run. The default is
1.

max-pending-list-length

The maximum number of pending dependencies scheduling will
allow before flushing the current state and starting over. Large functions
with few branches or calls can create excessively large lists which
needlessly consume memory and resources.

max-inline-insns-single

Several parameters control the tree inliner used in gcc.
This number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree inliner will
consider for inlining. This only affects functions declared inline and
methods implemented in a class declaration (C++). The default value is
450.

max-inline-insns-auto

When you use -finline-functions (included in
-O3), a lot of functions that would otherwise not be considered for
inlining by the compiler will be investigated. To those functions, a
different (more restrictive) limit compared to functions declared inline
can be applied. The default value is 90.

large-function-insns

The limit specifying really large functions. For functions
larger than this limit after inlining inlining is constrained by
--param large-function-growth. This parameter is useful primarily
to avoid extreme compilation time caused by non-linear algorithms used by
the backend. This parameter is ignored when -funit-at-a-time is not
used. The default value is 2700.

large-function-growth

Specifies maximal growth of large function caused by
inlining in percents. This parameter is ignored when
-funit-at-a-time is not used. The default value is 100 which limits
large function growth to 2.0 times the original size.

large-unit-insns

The limit specifying large translation unit. Growth caused
by inlining of units larger than this limit is limited by --param
inline-unit-growth. For small units this might be too tight (consider
unit consisting of function A that is inline and B that just calls A three
time. If B is small relative to A, the growth of unit is 300\% and yet
such inlining is very sane. For very large units consisting of small
inlininable functions however the overall unit growth limit is needed to
avoid exponential explosion of code size. Thus for smaller units, the size
is increased to --param large-unit-insns before applying --param
inline-unit-growth. The default is 10000

inline-unit-growth

Specifies maximal overall growth of the compilation unit
caused by inlining. This parameter is ignored when -funit-at-a-time
is not used. The default value is 50 which limits unit growth to 1.5 times
the original size.

max-inline-insns-recursive

max-inline-insns-recursive-auto

Specifies maximum number of instructions out-of-line copy
of self recursive inline function can grow into by performing recursive
inlining.

For functions declared inline --param max-inline-insns-recursive is
taken into account. For function not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-insns-recursive-auto is used. The
default value is 450.

max-inline-recursive-depth

max-inline-recursive-depth-auto

Specifies maximum recursion depth used by the recursive
inlining.

For functions declared inline --param max-inline-recursive-depth is
taken into account. For function not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-recursive-depth-auto is used. The
default value is 450.

min-inline-recursive-probability

Recursive inlining is profitable only for function having
deep recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or complexity of function
body to other optimizers.

When profile feedback is available (see -fprofile-generate) the
actual recursion depth can be guessed from probability that function will
recurse via given call expression. This parameter limits inlining only to
call expression whose probability exceeds given threshold (in percents).
The default value is 10.

inline-call-cost

Specify cost of call instruction relative to simple
arithmetics operations (having cost of 1). Increasing this cost
disqualifies inlining of non-leaf functions and at the same time increases
size of leaf function that is believed to reduce function size by being
inlined. In effect it increases amount of inlining for code having large
abstraction penalty (many functions that just pass the arguments to other
functions) and decrease inlining for code with low abstraction penalty.
The default value is 16.

max-unrolled-insns

The maximum number of instructions that a loop should have
if that loop is unrolled, and if the loop is unrolled, it determines how
many times the loop code is unrolled.

max-average-unrolled-insns

The maximum number of instructions biased by probabilities
of their execution that a loop should have if that loop is unrolled, and
if the loop is unrolled, it determines how many times the loop code is
unrolled.

max-unroll-times

The maximum number of unrollings of a single loop.

max-peeled-insns

The maximum number of instructions that a loop should have
if that loop is peeled, and if the loop is peeled, it determines how many
times the loop code is peeled.

max-peel-times

The maximum number of peelings of a single loop.

max-completely-peeled-insns

The maximum number of insns of a completely peeled
loop.

max-completely-peel-times

The maximum number of iterations of a loop to be suitable
for complete peeling.

max-unswitch-insns

The maximum number of insns of an unswitched loop.

max-unswitch-level

The maximum number of branches unswitched in a single
loop.

lim-expensive

The minimum cost of an expensive expression in the loop
invariant motion.

iv-consider-all-candidates-bound

Bound on number of candidates for induction variables below
that all candidates are considered for each use in induction variable
optimizations. Only the most relevant candidates are considered if there
are more candidates, to avoid quadratic time complexity.

iv-max-considered-uses

The induction variable optimizations give up on loops that
contain more induction variable uses.

iv-always-prune-cand-set-bound

If number of candidates in the set is smaller than this
value, we always try to remove unnecessary ivs from the set during its
optimization when a new iv is added to the set.

scev-max-expr-size

Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.

vect-max-version-checks

The maximum number of runtime checks that can be performed
when doing loop versioning in the vectorizer. See option
ftree-vect-loop-version for more information.

max-iterations-to-track

The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries to
evaluate.

hot-bb-count-fraction

Select fraction of the maximal count of repetitions of
basic block in program given basic block needs to have to be considered
hot.

hot-bb-frequency-fraction

Select fraction of the maximal frequency of executions of
basic block in function given basic block needs to have to be considered
hot

max-predicted-iterations

The maximum number of loop iterations we predict
statically. This is useful in cases where function contain single loop
with known bound and other loop with unknown. We predict the known number
of iterations correctly, while the unknown number of iterations average to
roughly 10. This means that the loop without bounds would appear
artificially cold relative to the other one.

tracer-dynamic-coverage

tracer-dynamic-coverage-feedback

This value is used to limit superblock formation once the
given percentage of executed instructions is covered. This limits
unnecessary code size expansion.

The tracer-dynamic-coverage-feedback is used only when profile
feedback is available. The real profiles (as opposed to statically
estimated ones) are much less balanced allowing the threshold to be larger
value.

tracer-max-code-growth

Stop tail duplication once code growth has reached given
percentage. This is rather hokey argument, as most of the duplicates will
be eliminated later in cross jumping, so it may be set to much higher
values than is the desired code growth.

tracer-min-branch-ratio

Stop reverse growth when the reverse probability of best
edge is less than this threshold (in percent).

tracer-min-branch-ratio

tracer-min-branch-ratio-feedback

Stop forward growth if the best edge do have probability
lower than this threshold.

Similarly to tracer-dynamic-coverage two values are present, one for
compilation for profile feedback and one for compilation without. The
value for compilation with profile feedback needs to be more conservative
(higher) in order to make tracer effective.

max-cse-path-length

Maximum number of basic blocks on path that cse considers.
The default is 10.

max-cse-insns

The maximum instructions CSE process before flushing. The
default is 1000.

global-var-threshold

Counts the number of function calls (n) and the
number of call-clobbered variables ( v). If nxv is
larger than this limit, a single artificial variable will be created to
represent all the call-clobbered variables at function call sites. This
artificial variable will then be made to alias every call-clobbered
variable. (done as "int * size_t" on the host machine; beware
overflow).

max-aliased-vops

Maximum number of virtual operands allowed to represent
aliases before triggering the alias grouping heuristic. Alias grouping
reduces compile times and memory consumption needed for aliasing at the
expense of precision loss in alias information.

ggc-min-expand

GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage by which the
garbage collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM
>= 1GB. If "getrlimit" is available, the notion of
"RAM" is the smallest of actual RAM and "RLIMIT_DATA"
or "RLIMIT_AS". If GCC is not able to calculate RAM on a
particular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection to
occur at every opportunity. This is extremely slow, but can be useful for
debugging.

ggc-min-heapsize

Minimum size of the garbage collector's heap before it
begins bothering to collect garbage. The first collection occurs after the
heap expands by ggc-min-expand% beyond ggc-min-heapsize.
Again, tuning this may improve compilation speed, and has no effect on
code generation.

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to
ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower
bound of 4096 (four megabytes) and an upper bound of 131072 (128
megabytes). If GCC is not able to calculate RAM on a particular platform,
the lower bound is used. Setting this parameter very large effectively
disables garbage collection. Setting this parameter and
ggc-min-expand to zero causes a full collection to occur at every
opportunity.

max-reload-search-insns

The maximum number of instruction reload should look
backward for equivalent register. Increasing values mean more aggressive
optimization, making the compile time increase with probably slightly
better performance. The default value is 100.

max-cselib-memory-locations

The maximum number of memory locations cselib should take
into account. Increasing values mean more aggressive optimization, making
the compile time increase with probably slightly better performance. The
default value is 500.

max-flow-memory-locations

Similar as max-cselib-memory-locations but for
dataflow liveness. The default value is 100.

reorder-blocks-duplicate

reorder-blocks-duplicate-feedback

Used by basic block reordering pass to decide whether to
use unconditional branch or duplicate the code on its destination. Code is
duplicated when its estimated size is smaller than this value multiplied
by the estimated size of unconditional jump in the hot spots of the
program.

The reorder-block-duplicate-feedback is used only when profile
feedback is available and may be set to higher values than
reorder-block-duplicate since information about the hot spots is
more accurate.

max-sched-ready-insns

The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first scheduling
pass. Increasing values mean more thorough searches, making the
compilation time increase with probably little benefit. The default value
is 100.

max-sched-region-blocks

The maximum number of blocks in a region to be considered
for interblock scheduling. The default value is 10.

max-sched-region-insns

The maximum number of insns in a region to be considered
for interblock scheduling. The default value is 100.

min-spec-prob

The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling. The default value is 40.

max-sched-extend-regions-iters

The maximum number of iterations through CFG to extend
regions. 0 - disable region extension, N - do at most N iterations. The
default value is 0.

max-sched-insn-conflict-delay

The maximum conflict delay for an insn to be considered for
speculative motion. The default value is 3.

sched-spec-prob-cutoff

The minimal probability of speculation success (in
percents), so that speculative insn will be scheduled. The default value
is 40.

max-last-value-rtl

The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register as last known
value of that register. The default is 10000.

integer-share-limit

Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its speed. This sets
the maximum value of a shared integer constant's. The default value is
256.

min-virtual-mappings

Specifies the minimum number of virtual mappings in the
incremental SSA updater that should be registered to trigger the virtual
mappings heuristic defined by virtual-mappings-ratio. The default value is
100.

virtual-mappings-ratio

If the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated, then the
incremental SSA updater switches to a full update for those symbols. The
default ratio is 3.

ssp-buffer-size

The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when -fstack-protection is used.

max-jump-thread-duplication-stmts

Maximum number of statements allowed in a block that needs
to be duplicated when threading jumps.

max-fields-for-field-sensitive

Maximum number of fields in a structure we will treat in a
field sensitive manner during pointer analysis.

Options Controlling the Preprocessor

These options control the C preprocessor, which is run on each C source file
before actual compilation.

If you use the -E option, nothing is done except preprocessing. Some of
these options make sense only together with -E because they cause the
preprocessor output to be unsuitable for actual compilation.

You can use -Wp,option to bypass
the compiler driver and pass option directly through to the
preprocessor. If option contains commas, it is split into multiple
options at the commas. However, many options are modified, translated or
interpreted by the compiler driver before being passed to the preprocessor,
and -Wp forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the options
instead.

-Xpreprocessoroption

Pass option as an option to the preprocessor. You
can use this to supply system-specific preprocessor options which GCC does
not know how to recognize.

If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the
argument.

-Dname

Predefine name as a macro, with definition 1.

-Dname=definition

The contents of definition are tokenized and
processed as if they appeared during translation phase three in a
#define directive. In particular, the definition will be truncated
by embedded newline characters.

If you are invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters such as
spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line, write its
argument list with surrounding parentheses before the equals sign (if
any). Parentheses are meaningful to most shells, so you will need to quote
the option. With sh and csh,
-D'name(args...)=definition'
works.

-D and -U options are processed in the order they are given
on the command line. All -imacrosfile and -includefile options are processed after all -D and -U
options.

-Uname

Cancel any previous definition of name, either built
in or provided with a -D option.

-undef

Do not predefine any system-specific or GCC-specific
macros. The standard predefined macros remain defined.

-Idir

Add the directory dir to the list of directories to
be searched for header files. Directories named by -I are searched
before the standard system include directories. If the directory
dir is a standard system include directory, the option is ignored
to ensure that the default search order for system directories and the
special treatment of system headers are not defeated .

-ofile

Write output to file. This is the same as specifying
file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you
must use -o to specify the output file.

-Wall

Turns on all optional warnings which are desirable for
normal code. At present this is -Wcomment, -Wtrigraphs,
-Wmultichar and a warning about integer promotion causing a change
of sign in "#if" expressions. Note that many of the
preprocessor's warnings are on by default and have no options to control
them.

-Wcomment

-Wcomments

Warn whenever a comment-start sequence /* appears in
a /* comment, or whenever a backslash-newline appears in a
// comment. (Both forms have the same effect.)

-Wtrigraphs

Most trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped newline (
??/ at the end of a line) can, by changing where the comment begins
or ends. Therefore, only trigraphs that would form escaped newlines
produce warnings inside a comment.

This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get trigraph
conversion without warnings, but get the other -Wall warnings, use
-trigraphs -Wall -Wno-trigraphs.

-Wtraditional

Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and problematic constructs which should be
avoided.

-Wimport

Warn the first time #import is used.

-Wundef

Warn whenever an identifier which is not a macro is
encountered in an #if directive, outside of defined. Such
identifiers are replaced with zero.

-Wunused-macros

Warn about macros defined in the main file that are unused.
A macro is used if it is expanded or tested for existence at least
once. The preprocessor will also warn if the macro has not been used at
the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in
include files are not warned about.

Note: If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as unused. To avoid the
warning in such a case, you might improve the scope of the macro's
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

-Wendif-labels

Warn whenever an #else or an #endif are
followed by text. This usually happens in code of the form

#if FOO
...
#else FOO
...
#endif FOO

The second and third "FOO" should be in comments, but often are
not in older programs. This warning is on by default.

-Werror

Make all warnings into hard errors. Source code which
triggers warnings will be rejected.

-Wsystem-headers

Issue warnings for code in system headers. These are
normally unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.

-w

Suppress all warnings, including those which GNU CPP issues
by default.

-pedantic

Issue all the mandatory diagnostics listed in the C
standard. Some of them are left out by default, since they trigger
frequently on harmless code.

-pedantic-errors

Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that GCC
issues without -pedantic but treats as warnings.

-M

Instead of outputting the result of preprocessing, output a
rule suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing the
object file name for that source file, a colon, and the names of all the
included files, including those coming from -include or
-imacros command line options.

Unless specified explicitly (with -MT or -MQ), the object file
name consists of the basename of the source file with any suffix replaced
with object file suffix. If there are many included files then the rule is
split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency rules
you should explicitly specify the dependency output file with -MF,
or use an environment variable like DEPENDENCIES_OUTPUT. Debug
output will still be sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings
with an implicit -w.

-MM

Like -M but do not mention header files that are
found in system header directories, nor header files that are included,
directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that header
will appear in -MM dependency output. This is a slight change in
semantics from GCC versions 3.0 and earlier.

-MFfile

When used with -M or -MM, specifies a file to
write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent
preprocessed output.

When used with the driver options -MD or -MMD, -MF
overrides the default dependency output file.

-MG

In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising an
error. The dependency filename is taken directly from the
"#include" directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders this
useless.

This feature is used in automatic updating of makefiles.

-MP

This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to depend on nothing.
These dummy rules work around errors make gives if you remove
header files without updating the Makefile to match.

This is typical output:

test.o: test.c test.h

test.h:

-MTtarget

Change the target of the rule emitted by dependency
generation. By default CPP takes the name of the main input file,
including any path, deletes any file suffix such as .c, and appends
the platform's usual object suffix. The result is the target.

An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

-MQtarget

Same as -MT, but it quotes any characters which are
special to Make. -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with
-MQ.

-MD

-MD is equivalent to -M -MFfile,
except that -E is not implied. The driver determines file
based on whether an -o option is given. If it is, the driver uses
its argument but with a suffix of .d, otherwise it take the
basename of the input file and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if used without
-E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a
dependency output file as a side-effect of the compilation process.

-MMD

Like -MD except mention only user header files, not
system header files.

-fpch-deps

When using precompiled headers, this flag will cause the
dependency-output flags to also list the files from the precompiled
header's dependencies. If not specified only the precompiled header would
be listed and not the files that were used to create it because those
files are not consulted when a precompiled header is used.

-fpch-preprocess

This option allows use of a precompiled header together
with -E. It inserts a special "#pragma", "#pragma
GCC pch_preprocess "<filename>"" in the output to
mark the place where the precompiled header was found, and its filename.
When -fpreprocessed is in use, GCC recognizes this
"#pragma" and loads the PCH.

This option is off by default, because the resulting preprocessed output is
only really suitable as input to GCC. It is switched on by
-save-temps.

You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a different
location. The filename may be absolute or it may be relative to GCC's
current directory.

-x c

-x c++

-x objective-c

-x assembler-with-cpp

Specify the source language: C, C++, Objective-C, or
assembly. This has nothing to do with standards conformance or extensions;
it merely selects which base syntax to expect. If you give none of these
options, cpp will deduce the language from the extension of the source
file: .c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.

Note: Previous versions of cpp accepted a -lang option which
selected both the language and the standards conformance level. This
option has been removed, because it conflicts with the -l
option.

-std=standard

-ansi

Specify the standard to which the code should conform.
Currently CPP knows about C and C++ standards; others may be added in the
future.

standard may be one of:

"iso9899:1990"

"c89"

The ISO C standard from 1990. c89 is the customary
shorthand for this version of the standard.

The -ansi option is equivalent to -std=c89.

"iso9899:199409"

The 1990 C standard, as amended in 1994.

"iso9899:1999"

"c99"

"iso9899:199x"

"c9x"

The revised ISO C standard, published in December 1999.
Before publication, this was known as C9X.

"gnu89"

The 1990 C standard plus GNU extensions. This is the
default.

"gnu99"

"gnu9x"

The 1999 C standard plus GNU extensions.

"c++98"

The 1998 ISO C++ standard plus amendments.

"gnu++98"

The same as -std=c++98 plus GNU extensions. This is
the default for C++ code.

-I-

Split the include path. Any directories specified with
-I options before -I- are searched only for headers
requested with "#include " file""; they
are not searched for "#include < file>". If
additional directories are specified with -I options after the
-I-, those directories are searched for all #include
directives.

In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for
"#include " file"". This option has been
deprecated.

-nostdinc

Do not search the standard system directories for header
files. Only the directories you have specified with -I options (and
the directory of the current file, if appropriate) are searched.

-nostdinc++

Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories. (This
option is used when building the C++ library.)

-includefile

Process file as if "#include
"file"" appeared as the first line of the primary source
file. However, the first directory searched for file is the
preprocessor's working directory instead of the directory
containing the main source file. If not found there, it is searched for in
the remainder of the "#include "..."" search chain as
normal.

If multiple -include options are given, the files are included in the
order they appear on the command line.

-imacrosfile

Exactly like -include, except that any output
produced by scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a header without
also processing its declarations.

All files specified by -imacros are processed before all files
specified by -include.

-idirafterdir

Search dir for header files, but do it after
all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include
directory.

-iprefixprefix

Specify prefix as the prefix for subsequent
-iwithprefix options. If the prefix represents a directory, you
should include the final /.

-iwithprefixdir

-iwithprefixbeforedir

Append dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include search
path. -iwithprefixbefore puts it in the same place -I would;
-iwithprefix puts it where -idirafter would.

-isysrootdir

This option is like the --sysroot option, but
applies only to header files. See the --sysroot option for more
information.

-imultilibdir

Use dir as a subdirectory of the directory
containing target-specific C++ headers.

-isystemdir

Search dir for header files, after all directories
specified by -I but before the standard system directories. Mark it
as a system directory, so that it gets the same special treatment as is
applied to the standard system directories.

-iquotedir

Search dir only for header files requested with
"#include " file""; they are not
searched for "#include < file>", before all
directories specified by -I and before the standard system
directories.

-fdollars-in-identifiers

Accept $ in identifiers.

-fextended-identifiers

Accept universal character names in identifiers. This
option is experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.

-fpreprocessed

Indicate to the preprocessor that the input file has
already been preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes comments, so
that you can pass a file preprocessed with -C to the compiler
without problems. In this mode the integrated preprocessor is little more
than a tokenizer for the front ends.

-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions
that GCC uses for preprocessed files created by -save-temps.

-ftabstop=width

Set the distance between tab stops. This helps the
preprocessor report correct column numbers in warnings or errors, even if
tabs appear on the line. If the value is less than 1 or greater than 100,
the option is ignored. The default is 8.

-fexec-charset=charset

Set the execution character set, used for string and
character constants. The default is UTF-8. charset can be any
encoding supported by the system's "iconv" library routine.

-fwide-exec-charset=charset

Set the wide execution character set, used for wide string
and character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". As with
-fexec-charset, charset can be any encoding supported by the
system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in
"wchar_t".

-finput-charset=charset

Set the input character set, used for translation from the
character set of the input file to the source character set used by GCC.
If the locale does not specify, or GCC cannot get this information from
the locale, the default is UTF-8. This can be overridden by either the
locale or this command line option. Currently the command line option
takes precedence if there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.

-fworking-directory

Enable generation of linemarkers in the preprocessor output
that will let the compiler know the current working directory at the time
of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working
directory followed by two slashes. GCC will use this directory, when it's
present in the preprocessed input, as the directory emitted as the current
working directory in some debugging information formats. This option is
implicitly enabled if debugging information is enabled, but this can be
inhibited with the negated form -fno-working-directory. If the
-P flag is present in the command line, this option has no effect,
since no "#line" directives are emitted whatsoever.

-fno-show-column

Do not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program that does not
understand the column numbers, such as dejagnu.

-Apredicate=answer

Make an assertion with the predicate predicate and
answer answer. This form is preferred to the older form -Apredicate(answer), which is still supported,
because it does not use shell special characters.

-A -predicate=answer

Cancel an assertion with the predicate predicate and
answer answer.

-dCHARS

CHARS is a sequence of one or more of the following
characters, and must not be preceded by a space. Other characters are
interpreted by the compiler proper, or reserved for future versions of
GCC, and so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.

M

Instead of the normal output, generate a list of
#define directives for all the macros defined during the execution
of the preprocessor, including predefined macros. This gives you a way of
finding out what is predefined in your version of the preprocessor.
Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

will show all the predefined macros.

D

Like M except in two respects: it does not
include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both kinds of
output go to the standard output file.

N

Like D, but emit only the macro names, not their
expansions.

I

Output #include directives in addition to the result
of preprocessing.

-P

Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor on
something that is not C code, and will be sent to a program which might be
confused by the linemarkers.

-C

Do not discard comments. All comments are passed through to
the output file, except for comments in processed directives, which are
deleted along with the directive.

You should be prepared for side effects when using -C; it causes the
preprocessor to treat comments as tokens in their own right. For example,
comments appearing at the start of what would be a directive line have the
effect of turning that line into an ordinary source line, since the first
token on the line is no longer a #.

-CC

Do not discard comments, including during macro expansion.
This is like -C, except that comments contained within macros are
also passed through to the output file where the macro is expanded.

In addition to the side-effects of the -C option, the -CC
option causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

-traditional-cpp

Try to imitate the behavior of old-fashioned C
preprocessors, as opposed to ISO C preprocessors.

-trigraphs

Process trigraph sequences. These are three-character
sequences, all starting with ??, that are defined by ISO C to stand
for single characters. For example, ??/ stands for \, so
'??/n' is a character constant for a newline. By default, GCC
ignores trigraphs, but in standard-conforming modes it converts them. See
the -std and -ansi options.

Enable special code to work around file systems which only
permit very short file names, such as MS-DOS.

--help

--target-help

Print text describing all the command line options instead
of preprocessing anything.

-v

Verbose mode. Print out GNU CPP's version number at the
beginning of execution, and report the final form of the include
path.

-H

Print the name of each header file used, in addition to
other normal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header file
is printed with ...x and a valid one with ...! .

-version

--version

Print out GNU CPP's version number. With one dash, proceed
to preprocess as normal. With two dashes, exit immediately.

Passing Options to the Assembler

You can pass options to the assembler.

-Wa,option

Pass option as an option to the assembler. If
option contains commas, it is split into multiple options at the
commas.

-Xassembleroption

Pass option as an option to the assembler. You can
use this to supply system-specific assembler options which GCC does not
know how to recognize.

If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the argument.

Options for Linking

These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not doing a
link step.

object-file-name

A file name that does not end in a special recognized
suffix is considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input to the
linker.

-c

-S

-E

If any of these options is used, then the linker is not
run, and object file names should not be used as arguments.

-llibrary

-llibrary

Search the library named library when linking. (The
second alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)

It makes a difference where in the command you write this option; the linker
searches and processes libraries and object files in the order they are
specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to
functions in z, those functions may not be loaded.

The linker searches a standard list of directories for the library, which is
actually a file named liblibrary.a. The linker then
uses this file as if it had been specified precisely by name.

The directories searched include several standard system directories plus
any that you specify with -L.

Normally the files found this way are library files---archive files whose
members are object files. The linker handles an archive file by scanning
through it for members which define symbols that have so far been
referenced but not defined. But if the file that is found is an ordinary
object file, it is linked in the usual fashion. The only difference
between using an -l option and specifying a file name is that
-l surrounds library with lib and .a and
searches several directories.

-lobjc

You need this special case of the -l option in order
to link an Objective-C or Objective-C++ program.

-nostartfiles

Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless -nostdlib
or -nodefaultlibs is used.

-nodefaultlibs

Do not use the standard system libraries when linking. Only
the libraries you specify will be passed to the linker. The standard
startup files are used normally, unless -nostartfiles is used. The
compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.

-nostdlib

Do not use the standard system startup files or libraries
when linking. No startup files and only the libraries you specify will be
passed to the linker. The compiler may generate calls to
"memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in
libc. These entry points should be supplied through some other mechanism
when this option is specified.

One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal
subroutines that GCC uses to overcome shortcomings of particular machines,
or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, __main, used to ensure C++
constructors will be called.)

-pie

Produce a position independent executable on targets which
support it. For predictable results, you must also specify the same set of
options that were used to generate code ( -fpie, -fPIE, or
model suboptions) when you specify this option.

-rdynamic

Pass the flag -export-dynamic to the ELF linker, on
targets that support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from
within a program.

-s

Remove all symbol table and relocation information from the
executable.

-static

On systems that support dynamic linking, this prevents
linking with the shared libraries. On other systems, this option has no
effect.

-shared

Produce a shared object which can then be linked with other
objects to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options that
were used to generate code ( -fpic, -fPIC, or model
suboptions) when you specify this option.[1]

-shared-libgcc

-static-libgcc

On systems that provide libgcc as a shared library,
these options force the use of either the shared or static version
respectively. If no shared version of libgcc was built when the
compiler was configured, these options have no effect.

There are several situations in which an application should use the shared
libgcc instead of the static version. The most common of these is
when the application wishes to throw and catch exceptions across different
shared libraries. In that case, each of the libraries as well as the
application itself should use the shared libgcc.

Therefore, the G++ and GCJ drivers automatically add -shared-libgcc
whenever you build a shared library or a main executable, because C++ and
Java programs typically use exceptions, so this is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may find
that they will not always be linked with the shared libgcc. If GCC
finds, at its configuration time, that you have a non-GNU linker or a GNU
linker that does not support option --eh-frame-hdr, it will link
the shared version of libgcc into shared libraries by default.
Otherwise, it will take advantage of the linker and optimize away the
linking with the shared version of libgcc, linking with the static
version of libgcc by default. This allows exceptions to propagate through
such shared libraries, without incurring relocation costs at library load
time.

However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
libgcc.

-symbolic

Bind references to global symbols when building a shared
object. Warn about any unresolved references (unless overridden by the
link editor option -Xlinker -z -Xlinker defs). Only a few systems
support this option.

-Xlinkeroption

Pass option as an option to the linker. You can use
this to supply system-specific linker options which GCC does not know how
to recognize.

If you want to pass an option that takes an argument, you must use
-Xlinker twice, once for the option and once for the argument. For
example, to pass -assert definitions, you must write -Xlinker
-assert -Xlinker definitions. It does not work to write -Xlinker
"-assert definitions", because this passes the entire string
as a single argument, which is not what the linker expects.

-Wl,option

Pass option as an option to the linker. If
option contains commas, it is split into multiple options at the
commas.

-usymbol

Pretend the symbol symbol is undefined, to force
linking of library modules to define it. You can use -u multiple
times with different symbols to force loading of additional library
modules.

Options for Directory Search

These options specify directories to search for header files, for libraries and
for parts of the compiler:

-Idir

Add the directory dir to the head of the list of
directories to be searched for header files. This can be used to override
a system header file, substituting your own version, since these
directories are searched before the system header file directories.
However, you should not use this option to add directories that contain
vendor-supplied system header files (use -isystem for that). If you
use more than one -I option, the directories are scanned in
left-to-right order; the standard system directories come after.

If a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I option
will be ignored. The directory will still be searched but as a system
directory at its normal position in the system include chain. This is to
ensure that GCC's procedure to fix buggy system headers and the ordering
for the include_next directive are not inadvertently changed. If you
really need to change the search order for system directories, use the
-nostdinc and/or -isystem options.

-iquotedir

Add the directory dir to the head of the list of
directories to be searched for header files only for the case of
#include"file"; they are not
searched for #include <file>, otherwise just
like -I.

-Ldir

Add directory dir to the list of directories to be
searched for -l.

-Bprefix

This option specifies where to find the executables,
libraries, include files, and data files of the compiler itself.

The compiler driver program runs one or more of the subprograms cpp,
cc1, as and ld. It tries prefix as a prefix
for each program it tries to run, both with and without
machine/version/.

For each subprogram to be run, the compiler driver first tries the -B
prefix, if any. If that name is not found, or if -B was not
specified, the driver tries two standard prefixes, which are
/usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those
results in a file name that is found, the unmodified program name is
searched for using the directories specified in your PATH
environment variable.

The compiler will check to see if the path provided by the -B refers
to a directory, and if necessary it will add a directory separator
character at the end of the path.

-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these options
into -L options for the linker. They also apply to includes files
in the preprocessor, because the compiler translates these options into
-isystem options for the preprocessor. In this case, the compiler
appends include to the prefix.

The run-time support file libgcc.a can also be searched for using the
-B prefix, if needed. If it is not found there, the two standard
prefixes above are tried, and that is all. The file is left out of the
link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the
environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the
range 0 to 9, then it will be replaced by [dir/]include. This is to
help with boot-strapping the compiler.

-specs=file

Process file after the compiler reads in the
standard specs file, in order to override the defaults that the
gcc driver program uses when determining what switches to pass to
cc1, cc1plus, as, ld, etc. More than one
-specs=file can be specified on the command line, and they
are processed in order, from left to right.

--sysroot=dir

Use dir as the logical root directory for headers
and libraries. For example, if the compiler would normally search for
headers in /usr/include and libraries in /usr/lib, it will
instead search dir/usr/include and
dir/usr/lib.

If you use both this option and the -isysroot option, then the
--sysroot option will apply to libraries, but the -isysroot
option will apply to header files.

The GNU linker (beginning with version 2.16) has the necessary support for
this option. If your linker does not support this option, the header file
aspect of --sysroot will still work, but the library aspect will
not.

-I-

This option has been deprecated. Please use -iquote
instead for -I directories before the -I- and remove the
-I-. Any directories you specify with -I options before the
-I- option are searched only for the case of #include
"file"; they are not searched for #include
<file>.

If additional directories are specified with -I options after the
-I-, these directories are searched for all #include
directives. (Ordinarily all-I directories are used this
way.)

In addition, the -I- option inhibits the use of the current directory
(where the current input file came from) as the first search directory for
#include "file". There is no way to
override this effect of -I-. With -I. you can specify
searching the directory which was current when the compiler was invoked.
That is not exactly the same as what the preprocessor does by default, but
it is often satisfactory.

-I- does not inhibit the use of the standard system directories for
header files. Thus, -I- and -nostdinc are independent.

Specifying Target Machine and Compiler Version

The usual way to run GCC is to run the executable called gcc, or
<machine>-gcc when cross-compiling, or
<machine>-gcc-<version> to run a version other than the one
that was installed last. Sometimes this is inconvenient, so GCC provides
options that will switch to another cross-compiler or version.

-bmachine

The argument machine specifies the target machine
for compilation.

The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For example, if a
cross-compiler was configured with configurearm-elf,
meaning to compile for an arm processor with elf binaries, then you would
specify -b arm-elf to run that cross compiler. Because there are
other options beginning with -b, the configuration must contain a
hyphen.

-Vversion

The argument version specifies which version of GCC
to run. This is useful when multiple versions are installed. For example,
version might be 4.0, meaning to run GCC version 4.0.

The -V and -b options work by running the
<machine>-gcc-<version> executable, so there's no real
reason to use them if you can just run that directly.

Hardware Models and Configurations

Earlier we discussed the standard option -b which chooses among different
installed compilers for completely different target machines, such as VAX vs.
68000 vs. 80386.

In addition, each of these target machine types can have its own special
options, starting with -m, to choose among various hardware models or
configurations---for example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any model or
configuration, according to the options specified.

Some configurations of the compiler also support additional special options,
usually for compatibility with other compilers on the same platform.

ARC Options

These options are defined for ARC implementations:

-EL

Compile code for little endian mode. This is the
default.

-EB

Compile code for big endian mode.

-mmangle-cpu

Prepend the name of the cpu to all public symbol names. In
multiple-processor systems, there are many ARC variants with different
instruction and register set characteristics. This flag prevents code
compiled for one cpu to be linked with code compiled for another. No
facility exists for handling variants that are "almost
identical". This is an all or nothing option.

-mcpu=cpu

Compile code for ARC variant cpu. Which variants are
supported depend on the configuration. All variants support
-mcpu=base, this is the default.

-mtext=text-section

-mdata=data-section

-mrodata=readonly-data-section

Put functions, data, and readonly data in
text-section, data-section, and readonly-data-section
respectively by default. This can be overridden with the
"section" attribute.

Generate a stack frame that is compliant with the ARM
Procedure Call Standard for all functions, even if this is not strictly
necessary for correct execution of the code. Specifying
-fomit-frame-pointer with this option will cause the stack frames
not to be generated for leaf functions. The default is
-mno-apcs-frame.

-mapcs

This is a synonym for -mapcs-frame.

-mthumb-interwork

Generate code which supports calling between the ARM and
Thumb instruction sets. Without this option the two instruction sets
cannot be reliably used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is generated when
-mthumb-interwork is specified.

-mno-sched-prolog

Prevent the reordering of instructions in the function
prolog, or the merging of those instruction with the instructions in the
function's body. This means that all functions will start with a
recognizable set of instructions (or in fact one of a choice from a small
set of different function prologues), and this information can be used to
locate the start if functions inside an executable piece of code. The
default is -msched-prolog.

-mhard-float

Generate output containing floating point instructions.
This is the default.

-msoft-float

Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
ARM targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation.

-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.

-mfloat-abi=name

Specifies which ABI to use for floating point values.
Permissible values are: soft, softfp and hard.

soft and hard are equivalent to -msoft-float and
-mhard-float respectively. softfp allows the generation of
floating point instructions, but still uses the soft-float calling
conventions.

-mlittle-endian

Generate code for a processor running in little-endian
mode. This is the default for all standard configurations.

-mbig-endian

Generate code for a processor running in big-endian mode;
the default is to compile code for a little-endian processor.

-mwords-little-endian

This option only applies when generating code for
big-endian processors. Generate code for a little-endian word order but a
big-endian byte order. That is, a byte order of the form 32107654.
Note: this option should only be used if you require compatibility with
code for big-endian ARM processors generated by versions of the compiler
prior to 2.8.

This option is very similar to the -mcpu= option,
except that instead of specifying the actual target processor type, and
hence restricting which instructions can be used, it specifies that GCC
should tune the performance of the code as if the target were of the type
specified in this option, but still choosing the instructions that it will
generate based on the cpu specified by a -mcpu= option. For some
ARM implementations better performance can be obtained by using this
option.

-march=name

This specifies the name of the target ARM architecture. GCC
uses this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction with or
instead of the -mcpu= option. Permissible names are: armv2,
armv2a, armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5te, armv6, armv6j,
iwmmxt, ep9312.

-mfpu=name

-mfpe=number

-mfp=number

This specifies what floating point hardware (or hardware
emulation) is available on the target. Permissible names are: fpa,
fpe2, fpe3, maverick, vfp. -mfp and
-mfpe are synonyms for -mfpu=fpenumber, for
compatibility with older versions of GCC.

If -msoft-float is specified this specifies the format of floating
point values.

-mstructure-size-boundary=n

The size of all structures and unions will be rounded up to
a multiple of the number of bits set by this option. Permissible values
are 8, 32 and 64. The default value varies for different toolchains. For
the COFF targeted toolchain the default value is 8. A value of 64 is only
allowed if the underlying ABI supports it.

Specifying the larger number can produce faster, more efficient code, but
can also increase the size of the program. Different values are
potentially incompatible. Code compiled with one value cannot necessarily
expect to work with code or libraries compiled with another value, if they
exchange information using structures or unions.

-mabort-on-noreturn

Generate a call to the function "abort" at the
end of a "noreturn" function. It will be executed if the
function tries to return.

-mlong-calls

-mno-long-calls

Tells the compiler to perform function calls by first
loading the address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the target
function will lie outside of the 64 megabyte addressing range of the
offset based version of subroutine call instruction.

Even if this switch is enabled, not all function calls will be turned into
long calls. The heuristic is that static functions, functions which have
the short-call attribute, functions that are inside the scope of a
#pragma no_long_calls directive and functions whose definitions
have already been compiled within the current compilation unit, will not
be turned into long calls. The exception to this rule is that weak
function definitions, functions with the long-call attribute or the
section attribute, and functions that are within the scope of a
#pragma long_calls directive, will always be turned into long
calls.

This feature is not enabled by default. Specifying -mno-long-calls
will restore the default behavior, as will placing the function calls
within the scope of a #pragmalong_calls_off directive. Note
these switches have no effect on how the compiler generates code to handle
function calls via function pointers.

-mnop-fun-dllimport

Disable support for the "dllimport"
attribute.

-msingle-pic-base

Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each function. The run-time
system is responsible for initializing this register with an appropriate
value before execution begins.

-mpic-register=reg

Specify the register to be used for PIC addressing. The
default is R10 unless stack-checking is enabled, when R9 is used.

-mcirrus-fix-invalid-insns

Insert NOPs into the instruction stream to in order to work
around problems with invalid Maverick instruction combinations. This
option is only valid if the -mcpu=ep9312 option has been used to
enable generation of instructions for the Cirrus Maverick floating point
co-processor. This option is not enabled by default, since the problem is
only present in older Maverick implementations. The default can be
re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.

-mpoke-function-name

Write the name of each function into the text section,
directly preceding the function prologue. The generated code is similar to
this:

When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then
looks at location "pc - 12" and the top 8 bits are set, then we
know that there is a function name embedded immediately preceding this
location and has length "((pc[-3]) & 0xff000000)".

-mthumb

Generate code for the 16-bit Thumb instruction set. The
default is to use the 32-bit ARM instruction set.

-mtpcs-frame

Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all non-leaf functions. (A leaf function is
one that does not call any other functions.) The default is
-mno-tpcs-frame.

-mtpcs-leaf-frame

Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-apcs-leaf-frame.

-mcallee-super-interworking

Gives all externally visible functions in the file being
compiled an ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions to be
called from non-interworking code.

-mcaller-super-interworking

Allows calls via function pointers (including virtual
functions) to execute correctly regardless of whether the target code has
been compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled.

-mtp=name

Specify the access model for the thread local storage
pointer. The valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which fetches the thread pointer
from "cp15" directly (supported in the arm6k architecture), and
auto, which uses the best available method for the selected
processor. The default setting is auto.

AVR Options

These options are defined for AVR implementations:

-mmcu=mcu

Specify ATMEL AVR instruction set or MCU type.

Instruction set avr1 is for the minimal AVR core, not supported by the C
compiler, only for assembler programs (MCU types: at90s1200, attiny10,
attiny11, attiny12, attiny15, attiny28).

Specify the initial stack address, which may be a symbol or
numeric value, __stack is the default.

-mno-interrupts

Generated code is not compatible with hardware interrupts.
Code size will be smaller.

-mcall-prologues

Functions prologues/epilogues expanded as call to
appropriate subroutines. Code size will be smaller.

-mno-tablejump

Do not generate tablejump insns which sometimes increase
code size.

-mtiny-stack

Change only the low 8 bits of the stack pointer.

-mint8

Assume int to be 8 bit integer. This affects the sizes of
all types: A char will be 1 byte, an int will be 1 byte, an long will be 2
bytes and long long will be 4 bytes. Please note that this option does not
comply to the C standards, but it will provide you with smaller code
size.

Blackfin Options

-momit-leaf-frame-pointer

Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions. The
option -fomit-frame-pointer removes the frame pointer for all
functions which might make debugging harder.

-mspecld-anomaly

When enabled, the compiler will ensure that the generated
code does not contain speculative loads after jump instructions. This
option is enabled by default.

When enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of memory.

-mno-low-64k

Assume that the program is arbitrarily large. This is the
default.

-mid-shared-library

Generate code that supports shared libraries via the
library ID method. This allows for execute in place and shared libraries
in an environment without virtual memory management. This option implies
-fPIC.

-mno-id-shared-library

Generate code that doesn't assume ID based shared libraries
are being used. This is the default.

-mshared-library-id=n

Specified the identification number of the ID based shared
library being compiled. Specifying a value of 0 will generate more compact
code, specifying other values will force the allocation of that number to
the current library but is no more space or time efficient than omitting
this option.

-mlong-calls

-mno-long-calls

Tells the compiler to perform function calls by first
loading the address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the target
function will lie outside of the 24 bit addressing range of the offset
based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls
will restore the default behavior. Note these switches have no effect on
how the compiler generates code to handle function calls via function
pointers.

CRIS Options

These options are defined specifically for the CRIS ports.

-march=architecture-type

-mcpu=architecture-type

Generate code for the specified architecture. The choices
for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and
ETRAX 100 LX. Default is v0 except for
cris-axis-linux-gnu, where the default is v10.

-mtune=architecture-type

Tune to architecture-type everything applicable
about the generated code, except for the ABI and the set of available
instructions. The choices for architecture-type are the same as for
-march=architecture-type.

-mmax-stack-frame=n

Warn when the stack frame of a function exceeds n
bytes.

-melinux-stacksize=n

Only available with the cris-axis-aout target.
Arranges for indications in the program to the kernel loader that the
stack of the program should be set to n bytes.

-metrax4

-metrax100

The options -metrax4 and -metrax100 are
synonyms for -march=v3 and -march=v8 respectively.

-mmul-bug-workaround

-mno-mul-bug-workaround

Work around a bug in the "muls" and
"mulu" instructions for CPU models where it applies. This option
is active by default.

-mpdebug

Enable CRIS-specific verbose debug-related information in
the assembly code. This option also has the effect to turn off the
#NO_APP formatted-code indicator to the assembler at the beginning
of the assembly file.

-mcc-init

Do not use condition-code results from previous
instruction; always emit compare and test instructions before use of
condition codes.

-mno-side-effects

Do not emit instructions with side-effects in addressing
modes other than post-increment.

-mstack-align

-mno-stack-align

-mdata-align

-mno-data-align

-mconst-align

-mno-const-align

These options (no-options) arranges (eliminate
arrangements) for the stack-frame, individual data and constants to be
aligned for the maximum single data access size for the chosen CPU model.
The default is to arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.

-m32-bit

-m16-bit

-m8-bit

Similar to the stack- data- and const-align options above,
these options arrange for stack-frame, writable data and constants to all
be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.

-mno-prologue-epilogue

-mprologue-epilogue

With -mno-prologue-epilogue, the normal function
prologue and epilogue that sets up the stack-frame are omitted and no
return instructions or return sequences are generated in the code. Use
this option only together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved registers must be saved,
or storage for local variable needs to be allocated.

-mno-gotplt

-mgotplt

With -fpic and -fPIC, don't generate (do
generate) instruction sequences that load addresses for functions from the
PLT part of the GOT rather than (traditional on other architectures) calls
to the PLT. The default is -mgotplt.

-maout

Legacy no-op option only recognized with the cris-axis-aout
target.

-melf

Legacy no-op option only recognized with the cris-axis-elf
and cris-axis-linux-gnu targets.

-melinux

Only recognized with the cris-axis-aout target, where it
selects a GNU/linux-like multilib, include files and instruction set for
-march=v8.

-mlinux

Legacy no-op option only recognized with the
cris-axis-linux-gnu target.

-sim

This option, recognized for the cris-axis-aout and
cris-axis-elf arranges to link with input-output functions from a
simulator library. Code, initialized data and zero-initialized data are
allocated consecutively.

-sim2

Like -sim, but pass linker options to locate
initialized data at 0x40000000 and zero-initialized data at
0x80000000.

CRX Options

These options are defined specifically for the CRX ports.

-mmac

Enable the use of multiply-accumulate instructions.
Disabled by default.

-mpush-args

Push instructions will be used to pass outgoing arguments
when functions are called. Enabled by default.

Darwin Options

These options are defined for all architectures running the Darwin operating
system.

FSF GCC on Darwin does not create "fat" object files; it will create
an object file for the single architecture that it was built to target.
Apple's GCC on Darwin does create "fat" files if multiple
-arch options are used; it does so by running the compiler or linker
multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or
i686) is determined by the flags that specify the ISA that GCC is
targetting, like -mcpu or -march. The
-force_cpusubtype_ALL option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The
assembler, as, will only permit instructions to be used that are valid
for the subtype of the file it is generating, so you cannot put 64-bit
instructions in an ppc750 object file. The linker for shared libraries,
/usr/bin/libtool, will fail and print an error if asked to create a
shared library with a less restrictive subtype than its input files (for
instance, trying to put a ppc970 object file in a ppc7400
library). The linker for executables, ld, will quietly give the
executable the most restrictive subtype of any of its input files.

-Fdir

Add the framework directory dir to the head of the
list of directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.

A framework directory is a directory with frameworks in it. A framework is a
directory with a "Headers" and/or
"PrivateHeaders" directory contained directly in it that
ends in ".framework". The name of a framework is the name
of this directory excluding the ".framework". Headers
associated with the framework are found in one of those two directories,
with "Headers" being searched first. A subframework is a
framework directory that is in a framework's "Frameworks"
directory. Includes of subframework headers can only appear in a header of
a framework that contains the subframework, or in a sibling subframework
header. Two subframeworks are siblings if they occur in the same
framework. A subframework should not have the same name as a framework, a
warning will be issued if this is violated. Currently a subframework
cannot have subframeworks, in the future, the mechanism may be extended to
support this. The standard frameworks can be found in
"/System/Library/Frameworks" and
"/Library/Frameworks". An example include looks like
"#include <Framework/header.h>", where Framework
denotes the name of the framework and header.h is found in the
"PrivateHeaders" or "Headers"
directory.

-gused

Emit debugging information for symbols that are used. For
STABS debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.

-gfull

Emit debugging information for all symbols and types.

-mmacosx-version-min=version

The earliest version of MacOS X that this executable will
run on is version. Typical values of version include 10.1,
10.2, and 10.3.9.

The default for this option is to make choices that seem to be most
useful.

Override the defaults for bool so that
sizeof(bool)==1. By default sizeof(bool) is 4 when
compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86,
so this option has no effect on x86.

Warning: The -mone-byte-bool switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to conform to a
non-default data model.

Loads all members of static archive libraries. See man
ld(1) for more information.

-arch_errors_fatal

Cause the errors having to do with files that have the
wrong architecture to be fatal.

-bind_at_load

Causes the output file to be marked such that the dynamic
linker will bind all undefined references when the file is loaded or
launched.

-bundle

Produce a Mach-o bundle format file. See man ld(1)
for more information.

-bundle_loaderexecutable

This option specifies the executable that will be
loading the build output file being linked. See man ld(1) for more
information.

-dynamiclib

When passed this option, GCC will produce a dynamic library
instead of an executable when linking, using the Darwin libtool
command.

-force_cpusubtype_ALL

This causes GCC's output file to have the ALL
subtype, instead of one controlled by the -mcpu or -march
option.

-allowable_clientclient_name

-client_name

-compatibility_version

-current_version

-dead_strip

-dependency-file

-dylib_file

-dylinker_install_name

-dynamic

-exported_symbols_list

-filelist

-flat_namespace

-force_flat_namespace

-headerpad_max_install_names

-image_base

-init

-install_name

-keep_private_externs

-multi_module

-multiply_defined

-multiply_defined_unused

-noall_load

-no_dead_strip_inits_and_terms

-nofixprebinding

-nomultidefs

-noprebind

-noseglinkedit

-pagezero_size

-prebind

-prebind_all_twolevel_modules

-private_bundle

-read_only_relocs

-sectalign

-sectobjectsymbols

-whyload

-seg1addr

-sectcreate

-sectobjectsymbols

-sectorder

-segaddr

-segs_read_only_addr

-segs_read_write_addr

-seg_addr_table

-seg_addr_table_filename

-seglinkedit

-segprot

-segs_read_only_addr

-segs_read_write_addr

-single_module

-static

-sub_library

-sub_umbrella

-twolevel_namespace

-umbrella

-undefined

-unexported_symbols_list

-weak_reference_mismatches

-whatsloaded

These options are passed to the Darwin linker. The Darwin
linker man page describes them in detail.

DEC Alpha Options

These -m options are defined for the DEC Alpha implementations:

-mno-soft-float

-msoft-float

Use (do not use) the hardware floating-point instructions
for floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point operations.
If you are compiling for an Alpha without floating-point operations, you
must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are
required to have floating-point registers.

-mfp-reg

-mno-fp-regs

Generate code that uses (does not use) the floating-point
register set. -mno-fp-regs implies -msoft-float. If the
floating-point register set is not used, floating point operands are
passed in integer registers as if they were integers and floating-point
results are passed in $0 instead of $f0. This is a non-standard calling
sequence, so any function with a floating-point argument or return value
called by code compiled with -mno-fp-regs must also be compiled
with that option.

A typical use of this option is building a kernel that does not use, and
hence need not save and restore, any floating-point registers.

-mieee

The Alpha architecture implements floating-point hardware
optimized for maximum performance. It is mostly compliant with the IEEE
floating point standard. However, for full compliance, software assistance
is required. This option generates code fully IEEE compliant code
except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro "_IEEE_FP"
is defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE values
such as not-a-number and plus/minus infinity. Other Alpha compilers call
this option -ieee_with_no_inexact.

-mieee-with-inexact

This is like -mieee except the generated code also
maintains the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to
"_IEEE_FP", "_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since
there is very little code that depends on the inexact-flag, you
should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.

-mfp-trap-mode=trap-mode

This option controls what floating-point related traps are
enabled. Other Alpha compilers call this option -fptmtrap-mode. The trap mode can be set to one of four values:

n

This is the default (normal) setting. The only traps that
are enabled are the ones that cannot be disabled in software (e.g.,
division by zero trap).

u

In addition to the traps enabled by n, underflow
traps are enabled as well.

su

Like u, but the instructions are marked to be safe
for software completion (see Alpha architecture manual for details).

sui

Like su, but inexact traps are enabled as well.

-mfp-rounding-mode=rounding-mode

Selects the IEEE rounding mode. Other Alpha compilers call
this option -fprmrounding-mode. The rounding-mode
can be one of:

n

Normal IEEE rounding mode. Floating point numbers are
rounded towards the nearest machine number or towards the even machine
number in case of a tie.

Dynamic rounding mode. A field in the floating point
control register ( fpcr, see Alpha architecture reference manual)
controls the rounding mode in effect. The C library initializes this
register for rounding towards plus infinity. Thus, unless your program
modifies the fpcr, d corresponds to round towards plus
infinity.

-mtrap-precision=trap-precision

In the Alpha architecture, floating point traps are
imprecise. This means without software assistance it is impossible to
recover from a floating trap and program execution normally needs to be
terminated. GCC can generate code that can assist operating system trap
handlers in determining the exact location that caused a floating point
trap. Depending on the requirements of an application, different levels of
precisions can be selected:

p

Program precision. This option is the default and means a
trap handler can only identify which program caused a floating point
exception.

f

Function precision. The trap handler can determine the
function that caused a floating point exception.

i

Instruction precision. The trap handler can determine the
exact instruction that caused a floating point exception.

Other Alpha compilers provide the equivalent options called -scope_safe
and -resumption_safe.

-mieee-conformant

This option marks the generated code as IEEE conformant.
You must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line .eflag
48 in the function prologue of the generated assembly file. Under DEC
Unix, this has the effect that IEEE-conformant math library routines will
be linked in.

-mbuild-constants

Normally GCC examines a 32- or 64-bit integer constant to
see if it can construct it from smaller constants in two or three
instructions. If it cannot, it will output the constant as a literal and
generate code to load it from the data segment at runtime.

Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).

You would typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory before
it can find the variables and constants in its own data segment.

-malpha-as

-mgas

Select whether to generate code to be assembled by the
vendor-supplied assembler ( -malpha-as) or by the GNU assembler
-mgas.

-mbwx

-mno-bwx

-mcix

-mno-cix

-mfix

-mno-fix

-mmax

-mno-max

Indicate whether GCC should generate code to use the
optional BWX, CIX, FIX and MAX instruction sets. The default is to use the
instruction sets supported by the CPU type specified via -mcpu=
option or that of the CPU on which GCC was built if none was
specified.

-mfloat-vax

-mfloat-ieee

Generate code that uses (does not use) VAX F and G floating
point arithmetic instead of IEEE single and double precision.

-mexplicit-relocs

-mno-explicit-relocs

Older Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros does not
allow optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark which
relocations should apply to which instructions. This option is mostly
useful for debugging, as GCC detects the capabilities of the assembler
when it is built and sets the default accordingly.

-msmall-data

-mlarge-data

When -mexplicit-relocs is in effect, static data is
accessed via gp-relative relocations. When -msmall-data is
used, objects 8 bytes long or smaller are placed in a small data
area (the ".sdata" and ".sbss" sections) and are
accessed via 16-bit relocations off of the $gp register. This limits the
size of the small data area to 64KB, but allows the variables to be
directly accessed via a single instruction.

The default is -mlarge-data. With this option the data area is
limited to just below 2GB. Programs that require more than 2GB of data
must use "malloc" or "mmap" to allocate the data in
the heap instead of in the program's data segment.

When -msmall-text is used, the compiler assumes that
the code of the entire program (or shared library) fits in 4MB, and is
thus reachable with a branch instruction. When -msmall-data is
used, the compiler can assume that all local symbols share the same $gp
value, and thus reduce the number of instructions required for a function
call from 4 to 1.

The default is -mlarge-text.

-mcpu=cpu_type

Set the instruction set and instruction scheduling
parameters for machine type cpu_type. You can specify either the
EV style name or the corresponding chip number. GCC supports
scheduling parameters for the EV4, EV5 and EV6 family of processors and
will choose the default values for the instruction set from the processor
you specify. If you do not specify a processor type, GCC will default to
the processor on which the compiler was built.

Supported values for cpu_type are

ev4

ev45

21064

Schedules as an EV4 and has no instruction set
extensions.

ev5

21164

Schedules as an EV5 and has no instruction set
extensions.

ev56

21164a

Schedules as an EV5 and supports the BWX extension.

pca56

21164pc

21164PC

Schedules as an EV5 and supports the BWX and MAX
extensions.

ev6

21264

Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.

ev67

21264a

Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX
extensions.

-mtune=cpu_type

Set only the instruction scheduling parameters for machine
type cpu_type. The instruction set is not changed.

-mmemory-latency=time

Sets the latency the scheduler should assume for typical
memory references as seen by the application. This number is highly
dependent on the memory access patterns used by the application and the
size of the external cache on the machine.

Valid options for time are

number

A decimal number representing clock cycles.

L1

L2

L3

main

The compiler contains estimates of the number of clock
cycles for "typical" EV4 & EV5 hardware for the Level 1, 2
& 3 caches (also called Dcache, Scache, and Bcache), as well as to
main memory. Note that L3 is only valid for EV5.

Do not try to dynamically allocate condition code
registers, only use "icc0" and "fcc0".

-mdword

Change ABI to use double word insns.

-mno-dword

Do not use double word instructions.

-mdouble

Use floating point double instructions.

-mno-double

Do not use floating point double instructions.

-mmedia

Use media instructions.

-mno-media

Do not use media instructions.

-mmuladd

Use multiply and add/subtract instructions.

-mno-muladd

Do not use multiply and add/subtract instructions.

-mfdpic

Select the FDPIC ABI, that uses function descriptors to
represent pointers to functions. Without any PIC/PIE-related options, it
implies -fPIE. With -fpic or -fpie, it assumes GOT
entries and small data are within a 12-bit range from the GOT base
address; with -fPIC or -fPIE, GOT offsets are computed with
32 bits.

-minline-plt

Enable inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no effect without
-mfdpic. It's enabled by default if optimizing for speed and
compiling for shared libraries (i.e., -fPIC or -fpic), or
when an optimization option such as -O3 or above is present in the
command line.

-mTLS

Assume a large TLS segment when generating thread-local
code.

-mtls

Do not assume a large TLS segment when generating
thread-local code.

-mgprel-ro

Enable the use of "GPREL" relocations in the
FDPIC ABI for data that is known to be in read-only sections. It's enabled
by default, except for -fpic or -fpie: even though it may
help make the global offset table smaller, it trades 1 instruction for 4.
With -fPIC or -fPIE, it trades 3 instructions for 4, one of
which may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win. If it is
not, -mno-gprel-ro can be used to disable it.

-multilib-library-pic

Link with the (library, not FD) pic libraries. It's implied
by -mlibrary-pic, as well as by -fPIC and -fpic
without -mfdpic. You should never have to use it explicitly.

-mlinked-fp

Follow the EABI requirement of always creating a frame
pointer whenever a stack frame is allocated. This option is enabled by
default and can be disabled with -mno-linked-fp.

-mlong-calls

Use indirect addressing to call functions outside the
current compilation unit. This allows the functions to be placed anywhere
within the 32-bit address space.

-malign-labels

Try to align labels to an 8-byte boundary by inserting nops
into the previous packet. This option only has an effect when VLIW packing
is enabled. It doesn't create new packets; it merely adds nops to existing
ones.

-mlibrary-pic

Generate position-independent EABI code.

-macc-4

Use only the first four media accumulator registers.

-macc-8

Use all eight media accumulator registers.

-mpack

Pack VLIW instructions.

-mno-pack

Do not pack VLIW instructions.

-mno-eflags

Do not mark ABI switches in e_flags.

-mcond-move

Enable the use of conditional-move instructions (default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-cond-move

Disable the use of conditional-move instructions.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mscc

Enable the use of conditional set instructions (default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-scc

Disable the use of conditional set instructions.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mcond-exec

Enable the use of conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-cond-exec

Disable the use of conditional execution.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mvliw-branch

Run a pass to pack branches into VLIW instructions
(default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-vliw-branch

Do not run a pass to pack branches into VLIW instructions.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mmulti-cond-exec

Enable optimization of "&&" and
"⎪⎪" in conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-multi-cond-exec

Disable optimization of "&&" and
"⎪⎪" in conditional execution.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mnested-cond-exec

Enable nested conditional execution optimizations
(default).

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-mno-nested-cond-exec

Disable nested conditional execution optimizations.

This switch is mainly for debugging the compiler and will likely be removed
in a future version.

-moptimize-membar

This switch removes redundant "membar"
instructions from the compiler generated code. It is enabled by
default.

-mno-optimize-membar

This switch disables the automatic removal of redundant
"membar" instructions from the generated code.

Use the GNU C library instead of uClibc. This is the
default except on *-*-linux-*uclibc* targets.

-muclibc

Use uClibc instead of the GNU C library. This is the
default on *-*-linux-*uclibc* targets.

H8/300 Options

These -m options are defined for the H8/300 implementations:

-mrelax

Shorten some address references at link time, when
possible; uses the linker option -relax.

-mh

Generate code for the H8/300H.

-ms

Generate code for the H8S.

-mn

Generate code for the H8S and H8/300H in the normal mode.
This switch must be used either with -mh or -ms.

-ms2600

Generate code for the H8S/2600. This switch must be used
with -ms.

-mint32

Make "int" data 32 bits by default.

-malign-300

On the H8/300H and H8S, use the same alignment rules as for
the H8/300. The default for the H8/300H and H8S is to align longs and
floats on 4 byte boundaries. -malign-300 causes them to be aligned
on 2 byte boundaries. This option has no effect on the H8/300.

HPPA Options

These -m options are defined for the HPPA family of computers:

-march=architecture-type

Generate code for the specified architecture. The choices
for architecture-type are 1.0 for PA 1.0, 1.1 for PA
1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered
architectures will run on higher numbered architectures, but not the other
way around.

-mpa-risc-1-0

-mpa-risc-1-1

-mpa-risc-2-0

Synonyms for -march=1.0, -march=1.1, and
-march=2.0 respectively.

-mbig-switch

Generate code suitable for big switch tables. Use this
option only if the assembler/linker complain about out of range branches
within a switch table.

-mjump-in-delay

Fill delay slots of function calls with unconditional jump
instructions by modifying the return pointer for the function call to be
the target of the conditional jump.

-mdisable-fpregs

Prevent floating point registers from being used in any
manner. This is necessary for compiling kernels which perform lazy context
switching of floating point registers. If you use this option and attempt
to perform floating point operations, the compiler will abort.

-mdisable-indexing

Prevent the compiler from using indexing address modes.
This avoids some rather obscure problems when compiling MIG generated code
under MACH.

-mno-space-regs

Generate code that assumes the target has no space
registers. This allows GCC to generate faster indirect calls and use
unscaled index address modes.

This option will not work in the presence of shared libraries or nested
functions.

-mfixed-range=register-range

Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges
can be specified separated by a comma.

-mlong-load-store

Generate 3-instruction load and store sequences as
sometimes required by the HP-UX 10 linker. This is equivalent to the
+k option to the HP compilers.

-mportable-runtime

Use the portable calling conventions proposed by HP for ELF
systems.

-mgas

Enable the use of assembler directives only GAS
understands.

-mschedule=cpu-type

Schedule code according to the constraints for the machine
type cpu-type. The choices for cpu-type are 7007100, 7100LC, 7200, 7300 and 8000.
Refer to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is
8000.

-mlinker-opt

Enable the optimization pass in the HP-UX linker. Note this
makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8
and HP-UX 9 linkers in which they give bogus error messages when linking
some programs.

-msoft-float

Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
HPPA targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation. The embedded target hppa1.1-*-pro does provide
software floating point support.

-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.

-msio

Generate the predefine, "_SIO", for server IO.
The default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and
"_WSIO", for workstation IO. These options are available under
HP-UX and HI-UX.

-mgnu-ld

Use GNU ld specific options. This passes -shared to
ld when building a shared library. It is the default when GCC is
configured, explicitly or implicitly, with the GNU linker. This option
does not have any affect on which ld is called, it only changes what
parameters are passed to that ld. The ld that is called is determined by
the --with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be printed
using which `gcc -print-prog-name=ld`. This option is only
available on the 64 bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.

-mhp-ld

Use HP ld specific options. This passes -b to ld
when building a shared library and passes +Accept TypeMismatch to
ld on all links. It is the default when GCC is configured, explicitly or
implicitly, with the HP linker. This option does not have any affect on
which ld is called, it only changes what parameters are passed to that ld.
The ld that is called is determined by the --with-ld configure
option, GCC's program search path, and finally by the user's PATH.
The linker used by GCC can be printed using which`gcc
-print-prog-name=ld`. This option is only available on the 64 bit
HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.

-mlong-calls

Generate code that uses long call sequences. This ensures
that a call is always able to reach linker generated stubs. The default is
to generate long calls only when the distance from the call site to the
beginning of the function or translation unit, as the case may be, exceeds
a predefined limit set by the branch type being used. The limits for
normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0
and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the
-ffunction-sections option, or when using the -mgas and
-mno-portable-runtime options together under HP-UX with the SOM
linker.

It is normally not desirable to use this option as it will degrade
performance. However, it may be useful in large applications, particularly
when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler
and linker, and the type of code being generated. The impact on systems
that support long absolute calls, and long pic symbol-difference or
pc-relative calls should be relatively small. However, an indirect call is
used on 32-bit ELF systems in pic code and it is quite long.

-munix=unix-std

Generate compiler predefines and select a startfile for the
specified UNIX standard. The choices for unix-std are 93,
95 and 98. 93 is supported on all HP-UX versions.
95 is available on HP-UX 10.10 and later. 98 is available on
HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00,
95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11
and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX"
and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o.
-munix=98 provides additional predefines for
"_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED",
"_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

It is important to note that this option changes the interfaces for
various library routines. It also affects the operational behavior of the
C library. Thus, extreme care is needed in using this option.

Library code that is intended to operate with more than one UNIX standard
must test, set and restore the variable __xpg4_extended_mask as
appropriate. Most GNU software doesn't provide this capability.

-nolibdld

Suppress the generation of link options to search libdld.sl
when the -static option is specified on HP-UX 10 and later.

-static

The HP-UX implementation of setlocale in libc has a
dependency on libdld.sl. There isn't an archive version of libdld.sl.
Thus, when the -static option is specified, special link options
are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link
with libdld.sl when the -static option is specified. This causes
the resulting binary to be dynamic. On the 64-bit port, the linkers
generate dynamic binaries by default in any case. The -nolibdld
option can be used to prevent the GCC driver from adding these link
options.

-threads

Add support for multithreading with the dce thread
library under HP-UX. This option sets flags for both the preprocessor and
linker.

Intel 386 and AMD x86-64 Options

These -m options are defined for the i386 and x86-64 family of computers:

-mtune=cpu-type

Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of available instructions.
The choices for cpu-type are:

generic

Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run, then you
should use the corresponding -mtune option instead of
-mtune=generic. But, if you do not know exactly what CPU users of
your application will have, then you should use this option.

As new processors are deployed in the marketplace, the behavior of this
option will change. Therefore, if you upgrade to a newer version of GCC,
the code generated option will change to reflect the processors that were
most common when that version of GCC was released.

There is no -march=generic option because -march indicates the
instruction set the compiler can use, and there is no generic instruction
set applicable to all processors. In contrast, -mtune indicates the
processor (or, in this case, collection of processors) for which the code
is optimized.

native

This selects the CPU to tune for at compilation time by
determining the processor type of the compiling machine. Using
-mtune=native will produce code optimized for the local machine
under the constraints of the selected instruction set. Using
-march=native will enable all instruction subsets supported by the
local machine (hence the result might not run on different machines).

i386

Original Intel's i386 CPU.

i486

Intel's i486 CPU. (No scheduling is implemented for this
chip.)

i586, pentium

Intel Pentium CPU with no MMX support.

pentium-mmx

Intel PentiumMMX CPU based on Pentium core with MMX
instruction set support.

pentiumpro

Intel PentiumPro CPU.

i686

Same as "generic", but when used as
"march" option, PentiumPro instruction set will be used, so the
code will run on all i686 family chips.

pentium2

Intel Pentium2 CPU based on PentiumPro core with MMX
instruction set support.

pentium3, pentium3m

Intel Pentium3 CPU based on PentiumPro core with MMX and
SSE instruction set support.

pentium-m

Low power version of Intel Pentium3 CPU with MMX, SSE and
SSE2 instruction set support. Used by Centrino notebooks.

pentium4, pentium4m

Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set
support.

prescott

Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2
and SSE3 instruction set support.

IDT Winchip C6 CPU, dealt in same way as i486 with
additional MMX instruction set support.

winchip2

IDT Winchip2 CPU, dealt in same way as i486 with additional
MMX and 3dNOW! instruction set support.

c3

Via C3 CPU with MMX and 3dNOW! instruction set support. (No
scheduling is implemented for this chip.)

c3-2

Via C3-2 CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)

While picking a specific cpu-type will schedule things appropriately for
that particular chip, the compiler will not generate any code that does not
run on the i386 without the -march=cpu-type option being
used.

-march=cpu-type

Generate instructions for the machine type cpu-type.
The choices for cpu-type are the same as for -mtune.
Moreover, specifying -march=cpu-type implies
-mtune=cpu-type.

-mcpu=cpu-type

A deprecated synonym for -mtune.

-m386

-m486

-mpentium

-mpentiumpro

These options are synonyms for -mtune=i386,
-mtune=i486, -mtune=pentium, and -mtune=pentiumpro
respectively. These synonyms are deprecated.

-mfpmath=unit

Generate floating point arithmetics for selected unit
unit. The choices for unit are:

387

Use the standard 387 floating point coprocessor present
majority of chips and emulated otherwise. Code compiled with this option
will run almost everywhere. The temporary results are computed in 80bit
precision instead of precision specified by the type resulting in slightly
different results compared to most of other chips. See
-ffloat-store for more detailed description.

This is the default choice for i386 compiler.

sse

Use scalar floating point instructions present in the SSE
instruction set. This instruction set is supported by Pentium3 and newer
chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The
earlier version of SSE instruction set supports only single precision
arithmetics, thus the double and extended precision arithmetics is still
done using 387. Later version, present only in Pentium4 and the future AMD
x86-64 chips supports double precision arithmetics too.

For the i386 compiler, you need to use -march=cpu-type,
-msse or -msse2 switches to enable SSE extensions and make
this option effective. For the x86-64 compiler, these extensions are
enabled by default.

The resulting code should be considerably faster in the majority of cases
and avoid the numerical instability problems of 387 code, but may break
some existing code that expects temporaries to be 80bit.

This is the default choice for the x86-64 compiler.

sse,387

Attempt to utilize both instruction sets at once. This
effectively double the amount of available registers and on chips with
separate execution units for 387 and SSE the execution resources too. Use
this option with care, as it is still experimental, because the GCC
register allocator does not model separate functional units well resulting
in instable performance.

-masm=dialect

Output asm instructions using selected dialect.
Supported choices are intel or att (the default one). Darwin
does not support intel.

-mieee-fp

-mno-ieee-fp

Control whether or not the compiler uses IEEE floating
point comparisons. These handle correctly the case where the result of a
comparison is unordered.

-msoft-float

Generate output containing library calls for floating
point. Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are used, but
this can't be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating point results in the 80387
register stack, some floating point opcodes may be emitted even if
-msoft-float is used.

-mno-fp-ret-in-387

Do not use the FPU registers for return values of
functions.

The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.

-mno-fancy-math-387

Some 387 emulators do not support the "sin",
"cos" and "sqrt" instructions for the 387. Specify
this option to avoid generating those instructions. This option is the
default on FreeBSD, OpenBSD and NetBSD. This option is overridden when
-march indicates that the target cpu will always have an FPU and so
the instruction will not need emulation. As of revision 2.6.1, these
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.

-malign-double

-mno-align-double

Control whether GCC aligns "double", "long
double", and "long long" variables on a two word boundary
or a one word boundary. Aligning "double" variables on a two
word boundary will produce code that runs somewhat faster on a
Pentium at the expense of more memory.

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures
containing the above types will be aligned differently than the published
application binary interface specifications for the 386 and will not be
binary compatible with structures in code compiled without that
switch.

-m96bit-long-double

-m128bit-long-double

These switches control the size of "long double"
type. The i386 application binary interface specifies the size to be 96
bits, so -m96bit-long-double is the default in 32 bit mode.

Modern architectures (Pentium and newer) would prefer "long
double" to be aligned to an 8 or 16 byte boundary. In arrays or
structures conforming to the ABI, this would not be possible. So
specifying a -m128bit-long-double will align "long
double" to a 16 byte boundary by padding the "long double"
with an additional 32 bit zero.

In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that "long double" is to be aligned on 16 byte
boundary.

Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a "long double".

Warning: if you override the default value for your target ABI, the
structures and arrays containing "long double" variables will
change their size as well as function calling convention for function
taking "long double" will be modified. Hence they will not be
binary compatible with arrays or structures in code compiled without that
switch.

-mmlarge-data-threshold=number

When -mcmodel=medium is specified, the data greater
than threshold are placed in large data section. This value must be
the same across all object linked into the binary and defaults to
65535.

-msvr3-shlib

-mno-svr3-shlib

Control whether GCC places uninitialized local variables
into the "bss" or "data" segments. -msvr3-shlib
places them into "bss". These options are meaningful only on
System V Release 3.

-mrtd

Use a different function-calling convention, in which
functions that take a fixed number of arguments return with the
"ret" num instruction, which pops their arguments while
returning. This saves one instruction in the caller since there is no need
to pop the arguments there.

You can specify that an individual function is called with this calling
sequence with the function attribute stdcall. You can also override
the -mrtd option by using the function attribute cdecl.

Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take
variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function
with too many arguments. (Normally, extra arguments are harmlessly
ignored.)

-mregparm=num

Control how many registers are used to pass integer
arguments. By default, no registers are used to pass arguments, and at
most 3 registers can be used. You can control this behavior for a specific
function by using the function attribute regparm.

Warning: if you use this switch, and num is nonzero, then you
must build all modules with the same value, including any libraries. This
includes the system libraries and startup modules.

-msseregparm

Use SSE register passing conventions for float and double
arguments and return values. You can control this behavior for a specific
function by using the function attribute sseregparm.

Warning: if you use this switch then you must build all modules with
the same value, including any libraries. This includes the system
libraries and startup modules.

-mstackrealign

Realign the stack at entry. On the Intel x86, the
-mstackrealign option will generate an alternate prologue and
epilogue that realigns the runtime stack. This supports mixing legacy
codes that keep a 4-byte aligned stack with modern codes that keep a
16-byte stack for SSE compatibility. The alternate prologue and epilogue
are slower and bigger than the regular ones, and the alternate prologue
requires an extra scratch register; this lowers the number of registers
available if used in conjunction with the "regparm" attribute.
The -mstackrealign option is incompatible with the nested function
prologue; this is considered a hard error. See also the attribute
"force_align_arg_pointer", applicable to individual
functions.

-mpreferred-stack-boundary=num

Attempt to keep the stack boundary aligned to a 2 raised to
num byte boundary. If -mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128 bits).

On Pentium and PentiumPro, "double" and "long double"
values should be aligned to an 8 byte boundary (see -malign-double)
or suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16 byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred stack
boundary from a function compiled with a lower preferred stack boundary
will most likely misalign the stack. It is recommended that libraries that
use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases
code size. Code that is sensitive to stack space usage, such as embedded
systems and operating system kernels, may want to reduce the preferred
alignment to -mpreferred-stack-boundary=2.

-mmmx

-mno-mmx

-msse

-mno-sse

-msse2

-mno-sse2

-msse3

-mno-sse3

-m3dnow

-mno-3dnow

These switches enable or disable the use of instructions in
the MMX, SSE, SSE2 or 3DNow! extended instruction sets. These extensions
are also available as built-in functions: see X86 Built-in
Functions, for details of the functions enabled and disabled by these
switches.

To have SSE/SSE2 instructions generated automatically from floating-point
code (as opposed to 387 instructions), see -mfpmath=sse.

These options will enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications which
perform runtime CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In particular, the
file containing the CPU detection code should be compiled without these
options.

-mpush-args

-mno-push-args

Use PUSH operations to store outgoing parameters. This
method is shorter and usually equally fast as method using SUB/MOV
operations and is enabled by default. In some cases disabling it may
improve performance because of improved scheduling and reduced
dependencies.

-maccumulate-outgoing-args

If enabled, the maximum amount of space required for
outgoing arguments will be computed in the function prologue. This is
faster on most modern CPUs because of reduced dependencies, improved
scheduling and reduced stack usage when preferred stack boundary is not
equal to 2. The drawback is a notable increase in code size. This switch
implies -mno-push-args.

-mthreads

Support thread-safe exception handling on Mingw32.
Code that relies on thread-safe exception handling must compile and link
all code with the -mthreads option. When compiling,
-mthreads defines -D_MT; when linking, it links in a special
thread helper library -lmingwthrd which cleans up per thread
exception handling data.

-mno-align-stringops

Do not align destination of inlined string operations. This
switch reduces code size and improves performance in case the destination
is already aligned, but GCC doesn't know about it.

-minline-all-stringops

By default GCC inlines string operations only when
destination is known to be aligned at least to 4 byte boundary. This
enables more inlining, increase code size, but may improve performance of
code that depends on fast memcpy, strlen and memset for short
lengths.

-momit-leaf-frame-pointer

Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions. The
option -fomit-frame-pointer removes the frame pointer for all
functions which might make debugging harder.

-mtls-direct-seg-refs

-mno-tls-direct-seg-refs

Controls whether TLS variables may be accessed with offsets
from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether
the thread base pointer must be added. Whether or not this is legal
depends on the operating system, and whether it maps the segment to cover
the entire TLS area.

For systems that use GNU libc, the default is on.

These -m switches are supported in addition to the above on AMD x86-64
processors in 64-bit environments.

-m32

-m64

Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32 bits and generates
code that runs on any i386 system. The 64-bit environment sets int to 32
bits and long and pointer to 64 bits and generates code for AMD's x86-64
architecture. For darwin only the -m64 option turns off the
-fno-pic and -mdynamic-no-pic options.

-mno-red-zone

Do not use a so called red zone for x86-64 code. The red
zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the
location of the stack pointer that will not be modified by signal or
interrupt handlers and therefore can be used for temporary data without
adjusting the stack pointer. The flag -mno-red-zone disables this
red zone.

-mcmodel=small

Generate code for the small code model: the program and its
symbols must be linked in the lower 2 GB of the address space. Pointers
are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.

-mcmodel=kernel

Generate code for the kernel code model. The kernel runs in
the negative 2 GB of the address space. This model has to be used for
Linux kernel code.

-mcmodel=medium

Generate code for the medium model: The program is linked
in the lower 2 GB of the address space but symbols can be located anywhere
in the address space. Programs can be statically or dynamically linked,
but building of shared libraries are not supported with the medium
model.

-mcmodel=large

Generate code for the large model: This model makes no
assumptions about addresses and sizes of sections. Currently GCC does not
implement this model.

IA-64 Options

These are the -m options defined for the Intel IA-64 architecture.

-mbig-endian

Generate code for a big endian target. This is the default
for HP-UX.

-mlittle-endian

Generate code for a little endian target. This is the
default for AIX5 and GNU/Linux.

-mgnu-as

-mno-gnu-as

Generate (or don't) code for the GNU assembler. This is the
default.

-mgnu-ld

-mno-gnu-ld

Generate (or don't) code for the GNU linker. This is the
default.

-mno-pic

Generate code that does not use a global pointer register.
The result is not position independent code, and violates the IA-64
ABI.

-mvolatile-asm-stop

-mno-volatile-asm-stop

Generate (or don't) a stop bit immediately before and after
volatile asm statements.

-mregister-names

-mno-register-names

Generate (or don't) in, loc, and out
register names for the stacked registers. This may make assembler output
more readable.

-mno-sdata

-msdata

Disable (or enable) optimizations that use the small data
section. This may be useful for working around optimizer bugs.

-mconstant-gp

Generate code that uses a single constant global pointer
value. This is useful when compiling kernel code.

-mauto-pic

Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.

-minline-float-divide-min-latency

Generate code for inline divides of floating point values
using the minimum latency algorithm.

-minline-float-divide-max-throughput

Generate code for inline divides of floating point values
using the maximum throughput algorithm.

-minline-int-divide-min-latency

Generate code for inline divides of integer values using
the minimum latency algorithm.

-minline-int-divide-max-throughput

Generate code for inline divides of integer values using
the maximum throughput algorithm.

Don't (or do) generate assembler code for the DWARF2 line
number debugging info. This may be useful when not using the GNU
assembler.

-mearly-stop-bits

-mno-early-stop-bits

Allow stop bits to be placed earlier than immediately
preceding the instruction that triggered the stop bit. This can improve
instruction scheduling, but does not always do so.

-mfixed-range=register-range

Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges
can be specified separated by a comma.

Add support for multithreading using the POSIX threads
library. This option sets flags for both the preprocessor and linker. It
does not affect the thread safety of object code produced by the compiler
or that of libraries supplied with it. These are HP-UX specific
flags.

-milp32

-mlp64

Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits. These are
HP-UX specific flags.

-mno-sched-br-data-spec

-msched-br-data-spec

(Dis/En)able data speculative scheduling before reload.
This will result in generation of the ld.a instructions and the
corresponding check instructions (ld.c / chk.a). The default is
'disable'.

-msched-ar-data-spec

-mno-sched-ar-data-spec

(En/Dis)able data speculative scheduling after reload. This
will result in generation of the ld.a instructions and the corresponding
check instructions (ld.c / chk.a). The default is 'enable'.

-mno-sched-control-spec

-msched-control-spec

(Dis/En)able control speculative scheduling. This feature
is available only during region scheduling (i.e. before reload). This will
result in generation of the ld.s instructions and the corresponding check
instructions chk.s . The default is 'disable'.

-msched-br-in-data-spec

-mno-sched-br-in-data-spec

(En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads before reload. This is
effective only with -msched-br-data-spec enabled. The default is
'enable'.

-msched-ar-in-data-spec

-mno-sched-ar-in-data-spec

(En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads after reload. This is
effective only with -msched-ar-data-spec enabled. The default is
'enable'.

-msched-in-control-spec

-mno-sched-in-control-spec

(En/Dis)able speculative scheduling of the instructions
that are dependent on the control speculative loads. This is effective
only with -msched-control-spec enabled. The default is
'enable'.

-msched-ldc

-mno-sched-ldc

(En/Dis)able use of simple data speculation checks ld.c .
If disabled, only chk.a instructions will be emitted to check data
speculative loads. The default is 'enable'.

-mno-sched-control-ldc

-msched-control-ldc

(Dis/En)able use of ld.c instructions to check control
speculative loads. If enabled, in case of control speculative load with no
speculatively scheduled dependent instructions this load will be emitted
as ld.sa and ld.c will be used to check it. The default is 'disable'.

-mno-sched-spec-verbose

-msched-spec-verbose

(Dis/En)able printing of the information about speculative
motions.

-mno-sched-prefer-non-data-spec-insns

-msched-prefer-non-data-spec-insns

If enabled, data speculative instructions will be chosen
for schedule only if there are no other choices at the moment. This will
make the use of the data speculation much more conservative. The default
is 'disable'.

-mno-sched-prefer-non-control-spec-insns

-msched-prefer-non-control-spec-insns

If enabled, control speculative instructions will be chosen
for schedule only if there are no other choices at the moment. This will
make the use of the control speculation much more conservative. The
default is 'disable'.

-mno-sched-count-spec-in-critical-path

-msched-count-spec-in-critical-path

If enabled, speculative dependencies will be considered
during computation of the instructions priorities. This will make the use
of the speculation a bit more conservative. The default is 'disable'.

M32C Options

-mcpu=name

Select the CPU for which code is generated. name may
be one of r8c for the R8C/Tiny series, m16c for the M16C (up
to /60) series, m32cm for the M16C/80 series, or m32c for
the M32C/80 series.

-msim

Specifies that the program will be run on the simulator.
This causes an alternate runtime library to be linked in which supports,
for example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own runtime
library for whatever I/O functions are needed.

-memregs=number

Specifies the number of memory-based pseudo-registers GCC
will use during code generation. These pseudo-registers will be used like
real registers, so there is a tradeoff between GCC's ability to fit the
code into available registers, and the performance penalty of using memory
instead of registers. Note that all modules in a program must be compiled
with the same value for this option. Because of that, you must not use
this option with the default runtime libraries gcc builds.

M32R/D Options

These -m options are defined for Renesas M32R/D architectures:

-m32r2

Generate code for the M32R/2.

-m32rx

Generate code for the M32R/X.

-m32r

Generate code for the M32R. This is the default.

-mmodel=small

Assume all objects live in the lower 16MB of memory (so
that their addresses can be loaded with the "ld24" instruction),
and assume all subroutines are reachable with the "bl"
instruction. This is the default.

The addressability of a particular object can be set with the
"model" attribute.

-mmodel=medium

Assume objects may be anywhere in the 32-bit address space
(the compiler will generate "seth/add3" instructions to load
their addresses), and assume all subroutines are reachable with the
"bl" instruction.

-mmodel=large

Assume objects may be anywhere in the 32-bit address space
(the compiler will generate "seth/add3" instructions to load
their addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler will generate the much slower
"seth/add3/jl" instruction sequence).

-msdata=none

Disable use of the small data area. Variables will be put
into one of .data, bss, or .rodata (unless the
"section" attribute has been specified). This is the default.

The small data area consists of sections .sdata and .sbss.
Objects may be explicitly put in the small data area with the
"section" attribute using one of these sections.

-msdata=sdata

Put small global and static data in the small data area,
but do not generate special code to reference them.

-msdata=use

Put small global and static data in the small data area,
and generate special instructions to reference them.

-Gnum

Put global and static objects less than or equal to
num bytes into the small data or bss sections instead of the normal
data or bss sections. The default value of num is 8. The
-msdata option must be set to one of sdata or use for
this option to have any effect.

All modules should be compiled with the same -Gnum value.
Compiling with different values of num may or may not work; if it
doesn't the linker will give an error message---incorrect code will not be
generated.

-mdebug

Makes the M32R specific code in the compiler display some
statistics that might help in debugging programs.

-malign-loops

Align all loops to a 32-byte boundary.

-mno-align-loops

Do not enforce a 32-byte alignment for loops. This is the
default.

-missue-rate=number

Issue number instructions per cycle. number
can only be 1 or 2.

-mbranch-cost=number

number can only be 1 or 2. If it is 1 then branches
will be preferred over conditional code, if it is 2, then the opposite
will apply.

-mflush-trap=number

Specifies the trap number to use to flush the cache. The
default is 12. Valid numbers are between 0 and 15 inclusive.

-mno-flush-trap

Specifies that the cache cannot be flushed by using a
trap.

-mflush-func=name

Specifies the name of the operating system function to call
to flush the cache. The default is _flush_cache, but a function
call will only be used if a trap is not available.

-mno-flush-func

Indicates that there is no OS function for flushing the
cache.

M680x0 Options

These are the -m options defined for the 68000 series. The default values
for these options depends on which style of 68000 was selected when the
compiler was configured; the defaults for the most common choices are given
below.

-m68000

-mc68000

Generate output for a 68000. This is the default when the
compiler is configured for 68000-based systems.

Use this option for microcontrollers with a 68000 or EC000 core, including
the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68020

-mc68020

Generate output for a 68020. This is the default when the
compiler is configured for 68020-based systems.

-m68881

Generate output containing 68881 instructions for floating
point. This is the default for most 68020 systems unless --nfp was
specified when the compiler was configured.

-m68030

Generate output for a 68030. This is the default when the
compiler is configured for 68030-based systems.

-m68040

Generate output for a 68040. This is the default when the
compiler is configured for 68040-based systems.

This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not
have code to emulate those instructions.

-m68060

Generate output for a 68060. This is the default when the
compiler is configured for 68060-based systems.

This option inhibits the use of 68020 and 68881/68882 instructions that have
to be emulated by software on the 68060. Use this option if your 68060
does not have code to emulate those instructions.

-mcpu32

Generate output for a CPU32. This is the default when the
compiler is configured for CPU32-based systems.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including
the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and
68360.

-m5200

Generate output for a 520X "coldfire" family cpu.
This is the default when the compiler is configured for 520X-based
systems.

Use this option for microcontroller with a 5200 core, including the MCF5202,
MCF5203, MCF5204 and MCF5202.

-mcfv4e

Generate output for a ColdFire V4e family cpu (e.g.
547x/548x). This includes use of hardware floating point
instructions.

-m68020-40

Generate output for a 68040, without using any of the new
instructions. This results in code which can run relatively efficiently on
either a 68020/68881 or a 68030 or a 68040. The generated code does use
the 68881 instructions that are emulated on the 68040.

-m68020-60

Generate output for a 68060, without using any of the new
instructions. This results in code which can run relatively efficiently on
either a 68020/68881 or a 68030 or a 68040. The generated code does use
the 68881 instructions that are emulated on the 68060.

-msoft-float

Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
m68k targets. Normally the facilities of the machine's usual C compiler
are used, but this can't be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets m68k-*-aout and
m68k-*-coff do provide software floating point support.

-mshort

Consider type "int" to be 16 bits wide, like
"short int". Additionally, parameters passed on the stack are
also aligned to a 16-bit boundary even on targets whose API mandates
promotion to 32-bit.

-mnobitfield

Do not use the bit-field instructions. The -m68000,
-mcpu32 and -m5200 options imply -mnobitfield.

-mbitfield

Do use the bit-field instructions. The -m68020
option implies -mbitfield. This is the default if you use a
configuration designed for a 68020.

-mrtd

Use a different function-calling convention, in which
functions that take a fixed number of arguments return with the
"rtd" instruction, which pops their arguments while returning.
This saves one instruction in the caller since there is no need to pop the
arguments there.

This calling convention is incompatible with the one normally used on Unix,
so you cannot use it if you need to call libraries compiled with the Unix
compiler.

Also, you must provide function prototypes for all functions that take
variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function
with too many arguments. (Normally, extra arguments are harmlessly
ignored.)

The "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

Warning: if you use the -malign-int switch, GCC will align
structures containing the above types differently than most published
application binary interface specifications for the m68k.

-mpcrel

Use the pc-relative addressing mode of the 68000 directly,
instead of using a global offset table. At present, this option implies
-fpic, allowing at most a 16-bit offset for pc-relative addressing.
-fPIC is not presently supported with -mpcrel, though this
could be supported for 68020 and higher processors.

-mno-strict-align

-mstrict-align

Do not (do) assume that unaligned memory references will be
handled by the system.

-msep-data

Generate code that allows the data segment to be located in
a different area of memory from the text segment. This allows for execute
in place in an environment without virtual memory management. This option
implies -fPIC.

-mno-sep-data

Generate code that assumes that the data segment follows
the text segment. This is the default.

-mid-shared-library

Generate code that supports shared libraries via the
library ID method. This allows for execute in place and shared libraries
in an environment without virtual memory management. This option implies
-fPIC.

-mno-id-shared-library

Generate code that doesn't assume ID based shared libraries
are being used. This is the default.

-mshared-library-id=n

Specified the identification number of the ID based shared
library being compiled. Specifying a value of 0 will generate more compact
code, specifying other values will force the allocation of that number to
the current library but is no more space or time efficient than omitting
this option.

M68hc1x Options

These are the -m options defined for the 68hc11 and 68hc12
microcontrollers. The default values for these options depends on which style
of microcontroller was selected when the compiler was configured; the defaults
for the most common choices are given below.

-m6811

-m68hc11

Generate output for a 68HC11. This is the default when the
compiler is configured for 68HC11-based systems.

-m6812

-m68hc12

Generate output for a 68HC12. This is the default when the
compiler is configured for 68HC12-based systems.

-m68S12

-m68hcs12

Generate output for a 68HCS12.

-mauto-incdec

Enable the use of 68HC12 pre and post auto-increment and
auto-decrement addressing modes.

-minmax

-nominmax

Enable the use of 68HC12 min and max instructions.

-mlong-calls

-mno-long-calls

Treat all calls as being far away (near). If calls are
assumed to be far away, the compiler will use the "call"
instruction to call a function and the "rtc" instruction for
returning.

-mshort

Consider type "int" to be 16 bits wide, like
"short int".

-msoft-reg-count=count

Specify the number of pseudo-soft registers which are used
for the code generation. The maximum number is 32. Using more pseudo-soft
register may or may not result in better code depending on the program.
The default is 4 for 68HC11 and 2 for 68HC12.

MCore Options

These are the -m options defined for the Motorola M*Core processors.

-mhardlit

-mno-hardlit

Inline constants into the code stream if it can be done in
two instructions or less.

-mdiv

-mno-div

Use the divide instruction. (Enabled by default).

-mrelax-immediate

-mno-relax-immediate

Allow arbitrary sized immediates in bit operations.

-mwide-bitfields

-mno-wide-bitfields

Always treat bit-fields as int-sized.

-m4byte-functions

-mno-4byte-functions

Force all functions to be aligned to a four byte
boundary.

-mcallgraph-data

-mno-callgraph-data

Emit callgraph information.

-mslow-bytes

-mno-slow-bytes

Prefer word access when reading byte quantities.

-mlittle-endian

-mbig-endian

Generate code for a little endian target.

-m210

-m340

Generate code for the 210 processor.

MIPS Options

-EB

Generate big-endian code.

-EL

Generate little-endian code. This is the default for
mips*el-*-* configurations.

In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be
written r.

GCC defines two macros based on the value of this option. The first is
_MIPS_ARCH, which gives the name of target architecture, as a
string. The second has the form _MIPS_ARCH_foo, where
foo is the capitalized value of _MIPS_ARCH. For example,
-march=r2000 will set _MIPS_ARCH to "r2000"
and define the macro _MIPS_ARCH_R2000.

Note that the _MIPS_ARCH macro uses the processor names given above.
In other words, it will have the full prefix and will not abbreviate
000 as k. In the case of from-abi, the macro names
the resolved architecture (either "mips1" or
"mips3"). It names the default architecture when no
-march option is given.

-mtune=arch

Optimize for arch. Among other things, this option
controls the way instructions are scheduled, and the perceived cost of
arithmetic operations. The list of arch values is the same as for
-march.

When this option is not used, GCC will optimize for the processor specified
by -march. By using -march and -mtune together, it is
possible to generate code that will run on a family of processors, but
optimize the code for one particular member of that family.

-mtune defines the macros _MIPS_TUNE and
_MIPS_TUNE_foo, which work in the same way as the
-march ones described above.

-mips1

Equivalent to -march=mips1.

-mips2

Equivalent to -march=mips2.

-mips3

Equivalent to -march=mips3.

-mips4

Equivalent to -march=mips4.

-mips32

Equivalent to -march=mips32.

-mips32r2

Equivalent to -march=mips32r2.

-mips64

Equivalent to -march=mips64.

-mips16

-mno-mips16

Generate (do not generate) MIPS16 code. If GCC is
targetting a MIPS32 or MIPS64 architecture, it will make use of the
MIPS16e ASE.

-mabi=32

-mabi=o64

-mabi=n32

-mabi=64

-mabi=eabi

Generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates
64-bit code when you select a 64-bit architecture, but you can use
-mgp32 to get 32-bit code instead.

For information about the O64 ABI, see <
http://gcc.gnu.org/projects/mipso64-abi.html>.

-mabicalls

-mno-abicalls

Generate (do not generate) code that is suitable for
SVR4-style dynamic objects. -mabicalls is the default for
SVR4-based systems.

-mshared

-mno-shared

Generate (do not generate) code that is fully
position-independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.

All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables to use absolute accesses
for locally-binding symbols. It can also use shorter GP initialization
sequences and generate direct calls to locally-defined functions. This
mode is selected by -mno-shared.

-mno-shared depends on binutils 2.16 or higher and generates objects
that can only be linked by the GNU linker. However, the option does not
affect the ABI of the final executable; it only affects the ABI of
relocatable objects. Using -mno-shared will generally make
executables both smaller and quicker.

-mshared is the default.

-mxgot

-mno-xgot

Lift (do not lift) the usual restrictions on the size of
the global offset table.

GCC normally uses a single instruction to load values from the GOT. While
this is relatively efficient, it will only work if the GOT is smaller than
about 64k. Anything larger will cause the linker to report an error such
as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs, although it will also be less
efficient, since it will take three instructions to fetch the value of a
global symbol.

Note that some linkers can create multiple GOTs. If you have such a linker,
you should only need to use -mxgot when a single object file
accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent
code.

-mgp32

Assume that general-purpose registers are 32 bits
wide.

-mgp64

Assume that general-purpose registers are 64 bits
wide.

-mfp32

Assume that floating-point registers are 32 bits wide.

-mfp64

Assume that floating-point registers are 64 bits wide.

-mhard-float

Use floating-point coprocessor instructions.

-msoft-float

Do not use floating-point coprocessor instructions.
Implement floating-point calculations using library calls instead.

-msingle-float

Assume that the floating-point coprocessor only supports
single-precision operations.

-mdouble-float

Assume that the floating-point coprocessor supports
double-precision operations. This is the default.

-mdsp

-mno-dsp

Use (do not use) the MIPS DSP ASE.

-mpaired-single

-mno-paired-single

Use (do not use) paired-single floating-point instructions.
This option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.

-mips3d

-mno-mips3d

Use (do not use) the MIPS-3D ASE. The option -mips3d
implies -mpaired-single.

-mlong64

Force "long" types to be 64 bits wide. See
-mlong32 for an explanation of the default and the way that the
pointer size is determined.

-mlong32

Force "long", "int", and pointer types
to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends
on the ABI. All the supported ABIs use 32-bit "int"s. The n64
ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use
32-bit "long"s. Pointers are the same size as "long"s,
or the same size as integer registers, whichever is smaller.

-msym32

-mno-sym32

Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in combination with
-mabi=64 and -mno-abicalls because it allows GCC to generate
shorter and faster references to symbolic addresses.

-Gnum

Put global and static items less than or equal to
num bytes into the small data or bss section instead of the normal
data or bss section. This allows the data to be accessed using a single
instruction.

All modules should be compiled with the same -Gnum
value.

-membedded-data

-mno-embedded-data

Allocate variables to the read-only data section first if
possible, then next in the small data section if possible, otherwise in
data. This gives slightly slower code than the default, but reduces the
amount of RAM required when executing, and thus may be preferred for some
embedded systems.

-muninit-const-in-rodata

-mno-uninit-const-in-rodata

Put uninitialized "const" variables in the
read-only data section. This option is only meaningful in conjunction with
-membedded-data.

-msplit-addresses

-mno-split-addresses

Enable (disable) use of the "%hi()" and
"%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards
compatibility.

-mexplicit-relocs

-mno-explicit-relocs

Use (do not use) assembler relocation operators when
dealing with symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.

-mexplicit-relocs is the default if GCC was configured to use an
assembler that supports relocation operators.

-mcheck-zero-division

-mno-check-zero-division

Trap (do not trap) on integer division by zero. The default
is -mcheck-zero-division.

-mdivide-traps

-mdivide-breaks

MIPS systems check for division by zero by generating
either a conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also, some
versions of the Linux kernel have a bug that prevents trap from generating
the proper signal ("SIGFPE"). Use -mdivide-traps to allow
conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at
configure time using --with-divide=breaks. Divide-by-zero checks
can be completely disabled using -mno-check-zero-division.

-mmemcpy

-mno-memcpy

Force (do not force) the use of "memcpy()" for
non-trivial block moves. The default is -mno-memcpy, which allows
GCC to inline most constant-sized copies.

-mlong-calls

-mno-long-calls

Disable (do not disable) use of the "jal"
instruction. Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256 megabyte segment.

This option has no effect on abicalls code. The default is
-mno-long-calls.

-mmad

-mno-mad

Enable (disable) use of the "mad",
"madu" and "mul" instructions, as provided by the
R4650 ISA.

-mfused-madd

-mno-fused-madd

Enable (disable) use of the floating point
multiply-accumulate instructions, when they are available. The default is
-mfused-madd.

When multiply-accumulate instructions are used, the intermediate product is
calculated to infinite precision and is not subject to the FCSR Flush to
Zero bit. This may be undesirable in some circumstances.

-nocpp

Tell the MIPS assembler to not run its preprocessor over
user assembler files (with a .s suffix) when assembling them.

-mfix-r4000

-mno-fix-r4000

Work around certain R4000 CPU errata:

-

A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer division.

-

A double-word or a variable shift may give an incorrect
result if executed while an integer multiplication is in progress.

-

An integer division may give an incorrect result if started
in a delay slot of a taken branch or a jump.

-mfix-r4400

-mno-fix-r4400

Work around certain R4400 CPU errata:

-

A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer division.

-mfix-vr4120

-mno-fix-vr4120

Work around certain VR4120 errata:

-

"dmultu" does not always produce the correct
result.

-

"div" and "ddiv" do not always produce
the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in
libgcc.a. At present, these functions are only provided by the
"mips64vr*-elf" configurations.

Other VR4120 errata require a nop to be inserted between certain pairs of
instructions. These errata are handled by the assembler, not by GCC
itself.

-mfix-vr4130

Work around the VR4130 "mflo"/"mfhi"
errata. The workarounds are implemented by the assembler rather than by
GCC, although GCC will avoid using "mflo" and "mfhi"
if the VR4130 "macc", "macchi", "dmacc" and
"dmacchi" instructions are available instead.

-mfix-sb1

-mno-fix-sb1

Work around certain SB-1 CPU core errata. (This flag
currently works around the SB-1 revision 2 "F1" and
"F2" floating point errata.)

-mflush-func=func

-mno-flush-func

Specifies the function to call to flush the I and D caches,
or to not call any such function. If called, the function must take the
same arguments as the common "_flush_func()", that is, the
address of the memory range for which the cache is being flushed, the size
of the memory range, and the number 3 (to flush both caches). The default
depends on the target GCC was configured for, but commonly is either
_flush_func or __cpu_flush.

-mbranch-likely

-mno-branch-likely

Enable or disable use of Branch Likely instructions,
regardless of the default for the selected architecture. By default,
Branch Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64
architectures and processors which implement those architectures; for
those, Branch Likely instructions will not be generated by default because
the MIPS32 and MIPS64 architectures specifically deprecate their use.

-mfp-exceptions

-mno-fp-exceptions

Specifies whether FP exceptions are enabled. This affects
how we schedule FP instructions for some processors. The default is that
FP exceptions are enabled.

For instance, on the SB-1, if FP exceptions are disabled, and we are
emitting 64-bit code, then we can use both FP pipes. Otherwise, we can
only use one FP pipe.

-mvr4130-align

-mno-vr4130-align

The VR4130 pipeline is two-way superscalar, but can only
issue two instructions together if the first one is 8-byte aligned. When
this option is enabled, GCC will align pairs of instructions that it
thinks should execute in parallel.

This option only has an effect when optimizing for the VR4130. It normally
makes code faster, but at the expense of making it bigger. It is enabled
by default at optimization level -O3.

MMIX Options

These options are defined for the MMIX:

-mlibfuncs

-mno-libfuncs

Specify that intrinsic library functions are being
compiled, passing all values in registers, no matter the size.

-mepsilon

-mno-epsilon

Generate floating-point comparison instructions that
compare with respect to the "rE" epsilon register.

-mabi=mmixware

-mabi=gnu

Generate code that passes function parameters and return
values that (in the called function) are seen as registers $0 and up, as
opposed to the GNU ABI which uses global registers $231 and up.

-mzero-extend

-mno-zero-extend

When reading data from memory in sizes shorter than 64
bits, use (do not use) zero-extending load instructions by default, rather
than sign-extending ones.

-mknuthdiv

-mno-knuthdiv

Make the result of a division yielding a remainder have the
same sign as the divisor. With the default, -mno-knuthdiv, the sign
of the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.

-mtoplevel-symbols

-mno-toplevel-symbols

Prepend (do not prepend) a : to all global symbols,
so the assembly code can be used with the "PREFIX" assembly
directive.

-melf

Generate an executable in the ELF format, rather than the
default mmo format used by the mmix simulator.

-mbranch-predict

-mno-branch-predict

Use (do not use) the probable-branch instructions, when
static branch prediction indicates a probable branch.

-mbase-addresses

-mno-base-addresses

Generate (do not generate) code that uses base
addresses. Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to be set up in a
global register. The register is used for one or more base address
requests within the range 0 to 255 from the value held in the register.
The generally leads to short and fast code, but the number of different
data items that can be addressed is limited. This means that a program
that uses lots of static data may require -mno-base-addresses.

-msingle-exit

-mno-single-exit

Force (do not force) generated code to have a single exit
point in each function.

MN10300 Options

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug

Generate code to avoid bugs in the multiply instructions
for the MN10300 processors. This is the default.

-mno-mult-bug

Do not generate code to avoid bugs in the multiply
instructions for the MN10300 processors.

-mam33

Generate code which uses features specific to the AM33
processor.

-mno-am33

Do not generate code which uses features specific to the
AM33 processor. This is the default.

-mreturn-pointer-on-d0

When generating a function which returns a pointer, return
the pointer in both "a0" and "d0". Otherwise, the
pointer is returned only in a0, and attempts to call such functions
without a prototype would result in errors. Note that this option is on by
default; use -mno-return-pointer-on-d0 to disable it.

-mno-crt0

Do not link in the C run-time initialization object
file.

-mrelax

Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute memory
addresses. This option only has an effect when used on the command line
for the final link step.

This option makes symbolic debugging impossible.

MT Options

These -m options are defined for Morpho MT architectures:

-march=cpu-type

Generate code that will run on cpu-type, which is
the name of a system representing a certain processor type. Possible
values for cpu-type are ms1-64-001, ms1-16-002,
ms1-16-003 and ms2.

When this option is not used, the default is -march=ms1-16-002.

-mbacc

Use byte loads and stores when generating code.

-mno-bacc

Do not use byte loads and stores when generating code.

-msim

Use simulator runtime

-mno-crt0

Do not link in the C run-time initialization object file
crti.o. Other run-time initialization and termination files such as
startup.o and exit.o are still included on the linker
command line.

PDP-11 Options

These options are defined for the PDP-11:

-mfpu

Use hardware FPP floating point. This is the default. (FIS
floating point on the PDP-11/40 is not supported.)

-msoft-float

Do not use hardware floating point.

-mac0

Return floating-point results in ac0 (fr0 in Unix assembler
syntax).

-mno-ac0

Return floating-point results in memory. This is the
default.

-m40

Generate code for a PDP-11/40.

-m45

Generate code for a PDP-11/45. This is the default.

-m10

Generate code for a PDP-11/10.

-mbcopy-builtin

Use inline "movmemhi" patterns for copying
memory. This is the default.

-mbcopy

Do not use inline "movmemhi" patterns for copying
memory.

-mint16

-mno-int32

Use 16-bit "int". This is the default.

-mint32

-mno-int16

Use 32-bit "int".

-mfloat64

-mno-float32

Use 64-bit "float". This is the default.

-mfloat32

-mno-float64

Use 32-bit "float".

-mabshi

Use "abshi2" pattern. This is the default.

-mno-abshi

Do not use "abshi2" pattern.

-mbranch-expensive

Pretend that branches are expensive. This is for
experimenting with code generation only.

-mbranch-cheap

Do not pretend that branches are expensive. This is the
default.

-msplit

Generate code for a system with split I&D.

-mno-split

Generate code for a system without split I&D. This is
the default.

-munix-asm

Use Unix assembler syntax. This is the default when
configured for pdp11-*-bsd.

-mdec-asm

Use DEC assembler syntax. This is the default when
configured for any PDP-11 target other than pdp11-*-bsd.

PowerPC Options

These are listed under

IBM RS/6000 and PowerPC Options

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpower

-mno-power

-mpower2

-mno-power2

-mpowerpc

-mno-powerpc

-mpowerpc-gpopt

-mno-powerpc-gpopt

-mpowerpc-gfxopt

-mno-powerpc-gfxopt

-mpowerpc64

-mno-powerpc64

-mmfcrf

-mno-mfcrf

-mpopcntb

-mno-popcntb

-mfprnd

-mno-fprnd

GCC supports two related instruction set architectures for
the RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture
of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx,
6xx, and follow-on microprocessors.

Neither architecture is a subset of the other. However there is a large
common subset of instructions supported by both. An MQ register is
included in processors supporting the POWER architecture.

You use these options to specify which instructions are available on the
processor you are using. The default value of these options is determined
when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use the
-mcpu=cpu_type option rather than the options listed above.

The -mpower option allows GCC to generate instructions that are found
only in the POWER architecture and to use the MQ register. Specifying
-mpower2 implies -power and also allows GCC to generate
instructions that are present in the POWER2 architecture but not the
original POWER architecture.

The -mpowerpc option allows GCC to generate instructions that are
found only in the 32-bit subset of the PowerPC architecture. Specifying
-mpowerpc-gpopt implies -mpowerpc and also allows GCC to use
the optional PowerPC architecture instructions in the General Purpose
group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the Graphics group,
including floating-point select.

The -mmfcrf option allows GCC to generate the move from condition
register field instruction implemented on the POWER4 processor and other
processors that support the PowerPC V2.01 architecture. The
-mpopcntb option allows GCC to generate the popcount and double
precision FP reciprocal estimate instruction implemented on the POWER5
processor and other processors that support the PowerPC V2.02
architecture. The -mfprnd option allows GCC to generate the FP
round to integer instructions implemented on the POWER5+ processor and
other processors that support the PowerPC V2.03 architecture.

The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and to
treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.

If you specify both -mno-power and -mno-powerpc, GCC will use
only the instructions in the common subset of both architectures plus some
special AIX common-mode calls, and will not use the MQ register.
Specifying both -mpower and -mpowerpc permits GCC to use any
instruction from either architecture and to allow use of the MQ register;
specify this for the Motorola MPC601.

-mnew-mnemonics

-mold-mnemonics

Select which mnemonics to use in the generated assembler
code. With -mnew-mnemonics, GCC uses the assembler mnemonics
defined for the PowerPC architecture. With -mold-mnemonics it uses
the assembler mnemonics defined for the POWER architecture. Instructions
defined in only one architecture have only one mnemonic; GCC uses that
mnemonic irrespective of which of these options is specified.

GCC defaults to the mnemonics appropriate for the architecture in use.
Specifying -mcpu=cpu_type sometimes overrides the value of
these option. Unless you are building a cross-compiler, you should
normally not specify either -mnew-mnemonics or
-mold-mnemonics, but should instead accept the default.

-mcpu=common selects a completely generic processor. Code generated
under this option will run on any POWER or PowerPC processor. GCC will use
only the instructions in the common subset of both architectures, and will
not use the MQ register. GCC assumes a generic processor model for
scheduling purposes.

The other options specify a specific processor. Code generated under those
options will run best on that processor, and may not run at all on others.

The -mcpu options automatically enable or disable the following
options: -maltivec, -mfprnd, -mhard-float,
-mmfcrf, -mmultiple, -mnew-mnemonics,
-mpopcntb, -mpower, -mpower2, -mpowerpc64,
-mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring,
-mmulhw, -mdlmzb. The particular options set for any
particular CPU will vary between compiler versions, depending on what
setting seems to produce optimal code for that CPU; it doesn't necessarily
reflect the actual hardware's capabilities. If you wish to set an
individual option to a particular value, you may specify it after the
-mcpu option, like -mcpu=970-mno-altivec.

On AIX, the -maltivec and -mpowerpc64 options are not enabled
or disabled by the -mcpu option at present because AIX does not
have full support for these options. You may still enable or disable them
individually if you're sure it'll work in your environment.

-mtune=cpu_type

Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type, register usage, or
choice of mnemonics, as -mcpu=cpu_type would. The same
values for cpu_type are used for -mtune as for -mcpu.
If both are specified, the code generated will use the architecture,
registers, and mnemonics set by -mcpu, but the scheduling
parameters set by -mtune.

-mswdiv

-mno-swdiv

Generate code to compute division as reciprocal estimate
and iterative refinement, creating opportunities for increased throughput.
This feature requires: optional PowerPC Graphics instruction set for
single precision and FRE instruction for double precision, assuming
divides cannot generate user-visible traps, and the domain values not
include Infinities, denormals or zero denominator.

-maltivec

-mno-altivec

Generate code that uses (does not use) AltiVec
instructions, and also enable the use of built-in functions that allow
more direct access to the AltiVec instruction set. You may also need to
set -mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.

-mvrsave

-mno-vrsave

Generate VRSAVE instructions when generating AltiVec
code.

-msecure-plt

Generate code that allows ld and ld.so to build executables
and shared libraries with non-exec .plt and .got sections. This is a
PowerPC 32-bit SYSV ABI option.

-mbss-plt

Generate code that uses a BSS .plt section that ld.so fills
in, and requires .plt and .got sections that are both writable and
executable. This is a PowerPC 32-bit SYSV ABI option.

-misel

-mno-isel

This switch enables or disables the generation of ISEL
instructions.

-misel=yes/no

This switch has been deprecated. Use -misel and
-mno-isel instead.

-mspe

-mno-spe

This switch enables or disables the generation of SPE simd
instructions.

-mspe=yes/no

This option has been deprecated. Use -mspe and
-mno-spe instead.

-mfloat-gprs=yes/single/double/no

-mfloat-gprs

This switch enables or disables the generation of floating
point operations on the general purpose registers for architectures that
support it.

The argument yes or single enables the use of single-precision
floating point operations.

The argument double enables the use of single and double-precision
floating point operations.

The argument no disables floating point operations on the general
purpose registers.

This option is currently only available on the MPC854x.

-m32

-m64

Generate code for 32-bit or 64-bit environments of Darwin
and SVR4 targets (including GNU/Linux). The 32-bit environment sets int,
long and pointer to 32 bits and generates code that runs on any PowerPC
variant. The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits, and generates code for PowerPC64, as for
-mpowerpc64.

-mfull-toc

-mno-fp-in-toc

-mno-sum-in-toc

-mminimal-toc

Modify generation of the TOC (Table Of Contents), which is
created for every executable file. The -mfull-toc option is
selected by default. In that case, GCC will allocate at least one TOC
entry for each unique non-automatic variable reference in your program.
GCC will also place floating-point constants in the TOC. However, only
16,384 entries are available in the TOC.

If you receive a linker error message that saying you have overflowed the
available TOC space, you can reduce the amount of TOC space used with the
-mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants
in the TOC and -mno-sum-in-toc forces GCC to generate code to
calculate the sum of an address and a constant at run-time instead of
putting that sum into the TOC. You may specify one or both of these
options. Each causes GCC to produce very slightly slower and larger code
at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these
options, specify -mminimal-toc instead. This option causes GCC to
make only one TOC entry for every file. When you specify this option, GCC
will produce code that is slower and larger but which uses extremely
little TOC space. You may wish to use this option only on files that
contain less frequently executed code.

Produce code that conforms more closely to IBM XL compiler
semantics when using AIX-compatible ABI. Pass floating-point arguments to
prototyped functions beyond the register save area (RSA) on the stack in
addition to argument FPRs. Do not assume that most significant double in
128-bit long double value is properly rounded when comparing values and
converting to double. Use XL symbol names for long double support
routines.

The AIX calling convention was extended but not initially documented to
handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. IBM XL
compilers access floating point arguments which do not fit in the RSA from
the stack when a subroutine is compiled without optimization. Because
always storing floating-point arguments on the stack is inefficient and
rarely needed, this option is not enabled by default and only is necessary
when calling subroutines compiled by IBM XL compilers without
optimization.

-mpe

Support IBM RS/6000 SPParallel Environment
(PE). Link an application written to use message passing with special
startup code to enable the application to run. The system must have PE
installed in the standard location ( /usr/lpp/ppe.poe/), or the
specs file must be overridden with the -specs= option to
specify the appropriate directory location. The Parallel Environment does
not support threads, so the -mpe option and the -pthread
option are incompatible.

-malign-natural

-malign-power

On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the
option -malign-natural overrides the ABI-defined alignment of
larger types, such as floating-point doubles, on their natural size-based
boundary. The option -malign-power instructs GCC to follow the
ABI-specified alignment rules. GCC defaults to the standard alignment
defined in the ABI.

On 64-bit Darwin, natural alignment is the default, and -malign-power
is not supported.

-msoft-float

-mhard-float

Generate code that does not use (uses) the floating-point
register set. Software floating point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.

-mmultiple

-mno-multiple

Generate code that uses (does not use) the load multiple
word instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not generated
on PowerPC systems. Do not use -mmultiple on little endian PowerPC
systems, since those instructions do not work when the processor is in
little endian mode. The exceptions are PPC740 and PPC750 which permit the
instructions usage in little endian mode.

-mstring

-mno-string

Generate code that uses (does not use) the load string
instructions and the store string word instructions to save multiple
registers and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do not use
-mstring on little endian PowerPC systems, since those instructions
do not work when the processor is in little endian mode. The exceptions
are PPC740 and PPC750 which permit the instructions usage in little endian
mode.

-mupdate

-mno-update

Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by default.
If you use -mno-update, there is a small window between the time
that the stack pointer is updated and the address of the previous frame is
stored, which means code that walks the stack frame across interrupts or
signals may get corrupted data.

-mfused-madd

-mno-fused-madd

Generate code that uses (does not use) the floating point
multiply and accumulate instructions. These instructions are generated by
default if hardware floating is used.

-mmulhw

-mno-mulhw

Generate code that uses (does not use) the half-word
multiply and multiply-accumulate instructions on the IBM 405 and 440
processors. These instructions are generated by default when targetting
those processors.

-mdlmzb

-mno-dlmzb

Generate code that uses (does not use) the string-search
dlmzb instruction on the IBM 405 and 440 processors. This
instruction is generated by default when targetting those processors.

-mno-bit-align

-mbit-align

On System V.4 and embedded PowerPC systems do not (do)
force structures and unions that contain bit-fields to be aligned to the
base type of the bit-field.

For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align, the
structure would be aligned to a 1 byte boundary and be one byte in
size.

-mno-strict-align

-mstrict-align

On System V.4 and embedded PowerPC systems do not (do)
assume that unaligned memory references will be handled by the
system.

-mrelocatable

-mno-relocatable

On embedded PowerPC systems generate code that allows (does
not allow) the program to be relocated to a different address at runtime.
If you use -mrelocatable on any module, all objects linked together
must be compiled with -mrelocatable or
-mrelocatable-lib.

-mrelocatable-lib

-mno-relocatable-lib

On embedded PowerPC systems generate code that allows (does
not allow) the program to be relocated to a different address at runtime.
Modules compiled with -mrelocatable-lib can be linked with either
modules compiled without -mrelocatable and -mrelocatable-lib
or with modules compiled with the -mrelocatable options.

-mno-toc

-mtoc

On System V.4 and embedded PowerPC systems do not (do)
assume that register 2 contains a pointer to a global area pointing to the
addresses used in the program.

-mlittle

-mlittle-endian

On System V.4 and embedded PowerPC systems compile code for
the processor in little endian mode. The -mlittle-endian option is
the same as -mlittle.

-mbig

-mbig-endian

On System V.4 and embedded PowerPC systems compile code for
the processor in big endian mode. The -mbig-endian option is the
same as -mbig.

-mdynamic-no-pic

On Darwin and Mac OS X systems, compile code so that it is
not relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.

-mprioritize-restricted-insns=priority

This option controls the priority that is assigned to
dispatch-slot restricted instructions during the second scheduling pass.
The argument priority takes the value 0/1/2 to assign
no/highest/second-highest priority to dispatch slot restricted
instructions.

-msched-costly-dep=dependence_type

This option controls which dependences are considered
costly by the target during instruction scheduling. The argument
dependence_type takes one of the following values: no: no
dependence is costly, all: all dependences are costly,
true_store_to_load: a true dependence from store to load is costly,
store_to_load: any dependence from store to load is costly,
number: any dependence which latency >= number is
costly.

-minsert-sched-nops=scheme

This option controls which nop insertion scheme will be
used during the second scheduling pass. The argument scheme takes
one of the following values: no: Don't insert nops. pad: Pad
with nops any dispatch group which has vacant issue slots, according to
the scheduler's grouping. regroup_exact: Insert nops to force
costly dependent insns into separate groups. Insert exactly as many nops
as needed to force an insn to a new group, according to the estimated
processor grouping. number: Insert nops to force costly dependent
insns into separate groups. Insert number nops to force an insn to
a new group.

-mcall-sysv

On System V.4 and embedded PowerPC systems compile code
using calling conventions that adheres to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor supplement. This
is the default unless you configured GCC using
powerpc-*-eabiaix.

-mcall-sysv-eabi

Specify both -mcall-sysv and -meabi
options.

-mcall-sysv-noeabi

Specify both -mcall-sysv and -mno-eabi
options.

-mcall-solaris

On System V.4 and embedded PowerPC systems compile code for
the Solaris operating system.

-mcall-linux

On System V.4 and embedded PowerPC systems compile code for
the Linux-based GNU system.

-mcall-gnu

On System V.4 and embedded PowerPC systems compile code for
the Hurd-based GNU system.

-mcall-netbsd

On System V.4 and embedded PowerPC systems compile code for
the NetBSD operating system.

Extend the current ABI with a particular extension, or
remove such extension. Valid values are altivec, no-altivec,
spe, no-spe, ibmlongdouble,
ieeelongdouble.

-mabi=spe

Extend the current ABI with SPE ABI extensions. This does
not change the default ABI, instead it adds the SPE ABI extensions to the
current ABI.

-mabi=no-spe

Disable Booke SPE ABI extensions for the current ABI.

-mabi=ibmlongdouble

Change the current ABI to use IBM extended precision long
double. This is a PowerPC 32-bit SYSV ABI option.

-mabi=ieeelongdouble

Change the current ABI to use IEEE extended precision long
double. This is a PowerPC 32-bit Linux ABI option.

-mprototype

-mno-prototype

On System V.4 and embedded PowerPC systems assume that all
calls to variable argument functions are properly prototyped. Otherwise,
the compiler must insert an instruction before every non prototyped call
to set or clear bit 6 of the condition code register ( CR) to
indicate whether floating point values were passed in the floating point
registers in case the function takes a variable arguments. With
-mprototype, only calls to prototyped variable argument functions
will set or clear the bit.

-msim

On embedded PowerPC systems, assume that the startup module
is called sim-crt0.o and that the standard C libraries are
libsim.a and libc.a. This is the default for
powerpc-*-eabisim. configurations.

-mmvme

On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libmvme.a
and libc.a.

-mads

On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libads.a
and libc.a.

-myellowknife

On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libyk.a
and libc.a.

-mvxworks

On System V.4 and embedded PowerPC systems, specify that
you are compiling for a VxWorks system.

-mwindiss

Specify that you are compiling for the WindISS simulation
environment.

-memb

On embedded PowerPC systems, set the PPC_EMB bit in
the ELF flags header to indicate that eabi extended relocations are
used.

-meabi

-mno-eabi

On System V.4 and embedded PowerPC systems do (do not)
adhere to the Embedded Applications Binary Interface (eabi) which is a set
of modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function
"__eabi" is called to from "main" to set up the eabi
environment, and the -msdata option can use both "r2" and
"r13" to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16 byte boundary, do
not call an initialization function from "main", and the
-msdata option will only use "r13" to point to a single
small data area. The -meabi option is on by default if you
configured GCC using one of the powerpc*-*-eabi* options.

-msdata=eabi

On System V.4 and embedded PowerPC systems, put small
initialized "const" global and static data in the .sdata2
section, which is pointed to by register "r2". Put small
initialized non-"const" global and static data in the
.sdata section, which is pointed to by register "r13".
Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the -mrelocatable
option. The -msdata=eabi option also sets the -memb
option.

-msdata=sysv

On System V.4 and embedded PowerPC systems, put small
global and static data in the .sdata section, which is pointed to
by register "r13". Put small uninitialized global and static
data in the .sbss section, which is adjacent to the .sdata
section. The -msdata=sysv option is incompatible with the
-mrelocatable option.

-msdata=default

-msdata

On System V.4 and embedded PowerPC systems, if
-meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.

-msdata-data

On System V.4 and embedded PowerPC systems, put small
global data in the .sdata section. Put small uninitialized global
data in the .sbss section. Do not use register "r13" to
address small data however. This is the default behavior unless other
-msdata options are used.

-msdata=none

-mno-sdata

On embedded PowerPC systems, put all initialized global and
static data in the .data section, and all uninitialized data in the
.bss section.

-Gnum

On embedded PowerPC systems, put global and static items
less than or equal to num bytes into the small data or bss sections
instead of the normal data or bss section. By default, num is 8.
The -Gnum switch is also passed to the linker. All modules
should be compiled with the same -Gnum value.

-mregnames

-mno-regnames

On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic forms.

-mlongcall

-mno-longcall

By default assume that all calls are far away so that a
longer more expensive calling sequence is required. This is required for
calls further than 32 megabytes (33,554,432 bytes) from the current
location. A short call will be generated if the compiler knows the call
cannot be that far away. This setting can be overridden by the
"shortcall" function attribute, or by "#pragma
longcall(0)".

Some linkers are capable of detecting out-of-range calls and generating glue
code on the fly. On these systems, long calls are unnecessary and generate
slower code. As of this writing, the AIX linker can do this, as can the
GNU linker for PowerPC/64. It is planned to add this feature to the GNU
linker for 32-bit PowerPC systems as well.

On Darwin/PPC systems, "#pragma longcall" will generate "jbsr
callee, L42", plus a "branch island" (glue code). The two
target addresses represent the callee and the "branch island".
The Darwin/PPC linker will prefer the first address and generate a
"bl callee" if the PPC "bl" instruction will reach the
callee directly; otherwise, the linker will generate "bl L42" to
call the "branch island". The "branch island" is
appended to the body of the calling function; it computes the full 32-bit
address of the callee and jumps to it.

On Mach-O (Darwin) systems, this option directs the compiler emit to the
glue for every direct call, and the Darwin linker decides whether to use
or discard it.

In the future, we may cause GCC to ignore all longcall specifications when
the linker is known to generate glue.

-pthread

Adds support for multithreading with the pthreads
library. This option sets flags for both the preprocessor and linker.

S/390 and zSeries Options

These are the -m options defined for the S/390 and zSeries architecture.

-mhard-float

-msoft-float

Use (do not use) the hardware floating-point instructions
and registers for floating-point operations. When -msoft-float is
specified, functions in libgcc.a will be used to perform
floating-point operations. When -mhard-float is specified, the
compiler generates IEEE floating-point instructions. This is the
default.

-mlong-double-64

-mlong-double-128

These switches control the size of "long double"
type. A size of 64bit makes the "long double" type equivalent to
the "double" type. This is the default.

-mbackchain

-mno-backchain

Store (do not store) the address of the caller's frame as
backchain pointer into the callee's stack frame. A backchain may be needed
to allow debugging using tools that do not understand DWARF-2 call frame
information. When -mno-packed-stack is in effect, the backchain
pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed into the
topmost word of the 96/160 byte register save area.

In general, code compiled with -mbackchain is call-compatible with
code compiled with -mmo-backchain; however, use of the backchain
for debugging purposes usually requires that the whole binary is built
with -mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.

The default is to not maintain the backchain.

-mpacked-stack

-mno-packed-stack

Use (do not use) the packed stack layout. When
-mno-packed-stack is specified, the compiler uses the all fields of
the 96/160 byte register save area only for their default purpose; unused
fields still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register save
area; unused space is reused for other purposes, allowing for more
efficient use of the available stack space. However, when
-mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is
always saved two words below the backchain.

As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run
time, not just for debugging purposes. Such code is not call-compatible
with code compiled with -mpacked-stack. Also, note that the
combination of -mbackchain, -mpacked-stack and
-mhard-float is not supported. In order to build a linux kernel use
-msoft-float.

The default is to not use the packed stack layout.

-msmall-exec

-mno-small-exec

Generate (or do not generate) code using the
"bras" instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed 64k. The default is
to use the "basr" instruction instead, which does not have this
limitation.

-m64

-m31

When -m31 is specified, generate code compliant to
the GNU/Linux for S/390 ABI. When -m64 is specified, generate code
compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular
to generate 64-bit instructions. For the s390 targets, the default
is -m31, while the s390x targets default to
-m64.

-mzarch

-mesa

When -mzarch is specified, generate code using the
instructions available on z/Architecture. When -mesa is specified,
generate code using the instructions available on ESA/390. Note that
-mesa is not possible with -m64. When generating code
compliant to the GNU/Linux for S/390 ABI, the default is -mesa.
When generating code compliant to the GNU/Linux for zSeries ABI, the
default is -mzarch.

-mmvcle

-mno-mvcle

Generate (or do not generate) code using the
"mvcle" instruction to perform block moves. When
-mno-mvcle is specified, use a "mvc" loop instead. This
is the default unless optimizing for size.

-mdebug

-mno-debug

Print (or do not print) additional debug information when
compiling. The default is to not print debug information.

-march=cpu-type

Generate code that will run on cpu-type, which is
the name of a system representing a certain processor type. Possible
values for cpu-type are g5, g6, z900, and
z990. When generating code using the instructions available on
z/Architecture, the default is -march=z900. Otherwise, the default
is -march=g5.

-mtune=cpu-type

Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of available instructions.
The list of cpu-type values is the same as for -march. The
default is the value used for -march.

-mtpf-trace

-mno-tpf-trace

Generate code that adds (does not add) in TPF OS specific
branches to trace routines in the operating system. This option is off by
default, even when compiling for the TPF OS.

-mfused-madd

-mno-fused-madd

Generate code that uses (does not use) the floating point
multiply and accumulate instructions. These instructions are generated by
default if hardware floating point is used.

-mwarn-framesize=framesize

Emit a warning if the current function exceeds the given
frame size. Because this is a compile time check it doesn't need to be a
real problem when the program runs. It is intended to identify functions
which most probably cause a stack overflow. It is useful to be used in an
environment with limited stack size e.g. the linux kernel.

-mwarn-dynamicstack

Emit a warning if the function calls alloca or uses
dynamically sized arrays. This is generally a bad idea with a limited
stack size.

-mstack-guard=stack-guard

-mstack-size=stack-size

These arguments always have to be used in conjunction. If
they are present the s390 back end emits additional instructions in the
function prologue which trigger a trap if the stack size is
stack-guard bytes above the stack-size (remember that the
stack on s390 grows downward). These options are intended to be used to
help debugging stack overflow problems. The additionally emitted code
causes only little overhead and hence can also be used in production like
systems without greater performance degradation. The given values have to
be exact powers of 2 and stack-size has to be greater than
stack-guard without exceeding 64k. In order to be efficient the
extra code makes the assumption that the stack starts at an address
aligned to the value given by stack-size.

Score Options

These options are defined for Score implementations:

-meb

Compile code for big endian mode. This is the default.

-mel

Compile code for little endian mode.

-mnhwloop

Disable generate bcnz instruction.

-muls

Enable generate unaligned load and store instruction.

-mmac

Enable the use of multiply-accumulate instructions.
Disabled by default.

-mscore5

Specify the SCORE5 as the target architecture.

-mscore5u

Specify the SCORE5U of the target architecture.

-mscore7

Specify the SCORE7 as the target architecture. This is the
default.

-mscore7d

Specify the SCORE7D as the target architecture.

SH Options

These -m options are defined for the SH implementations:

-m1

Generate code for the SH1.

-m2

Generate code for the SH2.

-m2e

Generate code for the SH2e.

-m3

Generate code for the SH3.

-m3e

Generate code for the SH3e.

-m4-nofpu

Generate code for the SH4 without a floating-point
unit.

-m4-single-only

Generate code for the SH4 with a floating-point unit that
only supports single-precision arithmetic.

-m4-single

Generate code for the SH4 assuming the floating-point unit
is in single-precision mode by default.

-m4

Generate code for the SH4.

-m4a-nofpu

Generate code for the SH4al-dsp, or for a SH4a in such a
way that the floating-point unit is not used.

-m4a-single-only

Generate code for the SH4a, in such a way that no
double-precision floating point operations are used.

-m4a-single

Generate code for the SH4a assuming the floating-point unit
is in single-precision mode by default.

-m4a

Generate code for the SH4a.

-m4al

Same as -m4a-nofpu, except that it implicitly passes
-dsp to the assembler. GCC doesn't generate any DSP instructions at
the moment.

-mb

Compile code for the processor in big endian mode.

-ml

Compile code for the processor in little endian mode.

-mdalign

Align doubles at 64-bit boundaries. Note that this changes
the calling conventions, and thus some functions from the standard C
library will not work unless you recompile it first with
-mdalign.

-mrelax

Shorten some address references at link time, when
possible; uses the linker option -relax.

-mbigtable

Use 32-bit offsets in "switch" tables. The
default is to use 16-bit offsets.

-mfmovd

Enable the use of the instruction "fmovd".

-mhitachi

Comply with the calling conventions defined by
Renesas.

-mrenesas

Comply with the calling conventions defined by
Renesas.

-mno-renesas

Comply with the calling conventions defined for GCC before
the Renesas conventions were available. This option is the default for all
targets of the SH toolchain except for sh-symbianelf.

-mnomacsave

Mark the "MAC" register as call-clobbered, even
if -mhitachi is given.

-mieee

Increase IEEE-compliance of floating-point code. At the
moment, this is equivalent to -fno-finite-math-only. When
generating 16 bit SH opcodes, getting IEEE-conforming results for
comparisons of NANs / infinities incurs extra overhead in every floating
point comparison, therefore the default is set to
-ffinite-math-only.

-misize

Dump instruction size and location in the assembly
code.

-mpadstruct

This option is deprecated. It pads structures to multiple
of 4 bytes, which is incompatible with the SH ABI.

Generate a library function call to invalidate instruction
cache entries, after fixing up a trampoline. This library function call
doesn't assume it can write to the whole memory address space. This is the
default when the target is "sh-*-linux*".

-multcost=number

Set the cost to assume for a multiply insn.

-mdiv=strategy

Set the division strategy to use for SHmedia code.
strategy must be one of: call, call2, fp, inv, inv:minlat, inv20u,
inv20l, inv:call, inv:call2, inv:fp . "fp" performs the
operation in floating point. This has a very high latency, but needs only
a few instructions, so it might be a good choice if your code has enough
easily exploitable ILP to allow the compiler to schedule the floating
point instructions together with other instructions. Division by zero
causes a floating point exception. "inv" uses integer operations
to calculate the inverse of the divisor, and then multiplies the dividend
with the inverse. This strategy allows cse and hoisting of the inverse
calculation. Division by zero calculates an unspecified result, but does
not trap. "inv:minlat" is a variant of "inv" where if
no cse / hoisting opportunities have been found, or if the entire
operation has been hoisted to the same place, the last stages of the
inverse calculation are intertwined with the final multiply to reduce the
overall latency, at the expense of using a few more instructions, and thus
offering fewer scheduling opportunities with other code. "call"
calls a library function that usually implements the inv:minlat strategy.
This gives high code density for m5-*media-nofpu compilations.
"call2" uses a different entry point of the same library
function, where it assumes that a pointer to a lookup table has already
been set up, which exposes the pointer load to cse / code hoisting
optimizations. "inv:call", "inv:call2" and
"inv:fp" all use the "inv" algorithm for initial code
generation, but if the code stays unoptimized, revert to the
"call", "call2", or "fp" strategies,
respectively. Note that the potentially-trapping side effect of division
by zero is carried by a separate instruction, so it is possible that all
the integer instructions are hoisted out, but the marker for the side
effect stays where it is. A recombination to fp operations or a call is
not possible in that case. "inv20u" and "inv20l" are
variants of the "inv:minlat" strategy. In the case that the
inverse calculation was nor separated from the multiply, they speed up
division where the dividend fits into 20 bits (plus sign where
applicable), by inserting a test to skip a number of operations in this
case; this test slows down the case of larger dividends. inv20u assumes
the case of a such a small dividend to be unlikely, and inv20l assumes it
to be likely.

-mdivsi3_libfunc=name

Set the name of the library function used for 32 bit signed
division to name. This only affect the name used in the call and
inv:call division strategies, and the compiler will still expect the same
sets of input/output/clobbered registers as if this option was not
present.

-madjust-unroll

Throttle unrolling to avoid thrashing target registers.
This option only has an effect if the gcc code base supports the
TARGET_ADJUST_UNROLL_MAX target hook.

-mindexed-addressing

Enable the use of the indexed addressing mode for
SHmedia32/SHcompact. This is only safe if the hardware and/or OS implement
32 bit wrap-around semantics for the indexed addressing mode. The
architecture allows the implementation of processors with 64 bit MMU,
which the OS could use to get 32 bit addressing, but since no current
hardware implementation supports this or any other way to make the indexed
addressing mode safe to use in the 32 bit ABI, the default is
-mno-indexed-addressing.

-mgettrcost=number

Set the cost assumed for the gettr instruction to
number. The default is 2 if -mpt-fixed is in effect, 100
otherwise.

-mpt-fixed

Assume pt* instructions won't trap. This will generally
generate better scheduled code, but is unsafe on current hardware. The
current architecture definition says that ptabs and ptrel trap when the
target anded with 3 is 3. This has the unintentional effect of making it
unsafe to schedule ptabs / ptrel before a branch, or hoist it out of a
loop. For example, __do_global_ctors, a part of libgcc that runs
constructors at program startup, calls functions in a list which is
delimited by -1. With the -mpt-fixed option, the ptabs will be done before
testing against -1. That means that all the constructors will be run a bit
quicker, but when the loop comes to the end of the list, the program
crashes because ptabs loads -1 into a target register. Since this option
is unsafe for any hardware implementing the current architecture
specification, the default is -mno-pt-fixed. Unless the user specifies a
specific cost with -mgettrcost, -mno-pt-fixed also implies
-mgettrcost=100; this deters register allocation using target
registers for storing ordinary integers.

-minvalid-symbols

Assume symbols might be invalid. Ordinary function symbols
generated by the compiler will always be valid to load with
movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker
tricks it is possible to generate symbols that will cause ptabs / ptrel to
trap. This option is only meaningful when -mno-pt-fixed is in
effect. It will then prevent cross-basic-block cse, hoisting and most
scheduling of symbol loads. The default is
-mno-invalid-symbols.

SPARC Options

These -m options are supported on the SPARC:

-mno-app-regs

-mapp-regs

Specify -mapp-regs to generate output using the
global registers 2 through 4, which the SPARC SVR4 ABI reserves for
applications. This is the default.

To be fully SVR4 ABI compliant at the cost of some performance loss, specify
-mno-app-regs. You should compile libraries and system software
with this option.

-mfpu

-mhard-float

Generate output containing floating point instructions.
This is the default.

-mno-fpu

-msoft-float

Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all
SPARC targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating point support.

-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.

Generate output containing library calls for quad-word
(long double) floating point instructions. The functions called are those
specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware
support for the quad-word floating point instructions. They all invoke a
trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler
overhead, this is much slower than calling the ABI library routines. Thus
the -msoft-quad-float option is the default.

-mno-unaligned-doubles

-munaligned-doubles

Assume that doubles have 8 byte alignment. This is the
default.

With -munaligned-doubles, GCC assumes that doubles have 8 byte
alignment only if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility problems with code
generated by other compilers. It is not the default because it results in
a performance loss, especially for floating point code.

-mno-faster-structs

-mfaster-structs

With -mfaster-structs, the compiler assumes that
structures should have 8 byte alignment. This enables the use of pairs of
"ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st"
pairs. However, the use of this changed alignment directly violates the
SPARC ABI. Thus, it's intended only for use on targets where the developer
acknowledges that their resulting code will not be directly in line with
the rules of the ABI.

-mimpure-text

-mimpure-text, used in addition to -shared,
tells the compiler to not pass -z text to the linker when linking a
shared object. Using this option, you can link position-dependent code
into a shared object.

-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error message. However,
the necessary relocations will trigger copy-on-write, and the shared
object is not actually shared across processes. Instead of using
-mimpure-text, you should compile all source code with -fpic
or -fPIC.

By default (unless configured otherwise), GCC generates code for the V7
variant of the SPARC architecture. With -mcpu=cypress, the compiler
additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the
older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the compiler emits
the integer multiply and integer divide instructions which exist in
SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler
additionally optimizes it for the SuperSPARC chip, as used in the
SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of
the SPARC architecture. This adds the integer multiply, integer divide
step and scan ("ffs") instructions which exist in SPARClite but
not in SPARC-V7. With -mcpu=f930, the compiler additionally
optimizes it for the Fujitsu MB86930 chip, which is the original
SPARClite, with no FPU. With -mcpu=f934, the compiler additionally
optimizes it for the Fujitsu MB86934 chip, which is the more recent
SPARClite with FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant of
the SPARC architecture. This adds the integer multiply,
multiply/accumulate, integer divide step and scan ("ffs")
instructions which exist in SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move
instructions, 3 additional floating-point condition code registers and
conditional move instructions. With -mcpu=ultrasparc, the compiler
additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally optimizes it for the
Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips.

-mtune=cpu_type

Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type would.

The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those that
select a particular cpu implementation. Those are cypress,
supersparc, hypersparc, f930, f934,
sparclite86x, tsc701, ultrasparc, ultrasparc3,
and niagara.

-mv8plus

-mno-v8plus

With -mv8plus, GCC generates code for the SPARC-V8+
ABI. The difference from the V8 ABI is that the global and out registers
are considered 64-bit wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.

-mvis

-mno-vis

With -mvis, GCC generates code that takes advantage
of the UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.

These -m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:

-mlittle-endian

Generate code for a processor running in little-endian
mode. It is only available for a few configurations and most notably not
on Solaris and Linux.

-m32

-m64

Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.

-mcmodel=medlow

Generate code for the Medium/Low code model: 64-bit
addresses, programs must be linked in the low 32 bits of memory. Programs
can be statically or dynamically linked.

-mcmodel=medmid

Generate code for the Medium/Middle code model: 64-bit
addresses, programs must be linked in the low 44 bits of memory, the text
and data segments must be less than 2GB in size and the data segment must
be located within 2GB of the text segment.

-mcmodel=medany

Generate code for the Medium/Anywhere code model: 64-bit
addresses, programs may be linked anywhere in memory, the text and data
segments must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.

-mcmodel=embmedany

Generate code for the Medium/Anywhere code model for
embedded systems: 64-bit addresses, the text and data segments must be
less than 2GB in size, both starting anywhere in memory (determined at
link time). The global register %g4 points to the base of the data
segment. Programs are statically linked and PIC is not supported.

-mstack-bias

-mno-stack-bias

With -mstack-bias, GCC assumes that the stack
pointer, and frame pointer if present, are offset by -2047 which must be
added back when making stack frame references. This is the default in
64-bit mode. Otherwise, assume no such offset is present.

These switches are supported in addition to the above on Solaris:

-threads

Add support for multithreading using the Solaris threads
library. This option sets flags for both the preprocessor and linker. This
option does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it.

-pthreads

Add support for multithreading using the POSIX threads
library. This option sets flags for both the preprocessor and linker. This
option does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it.

-pthread

This is a synonym for -pthreads.

Options for System V

These additional options are available on System V Release 4 for compatibility
with other compilers on those systems:

-G

Create a shared object. It is recommended that
-symbolic or -shared be used instead.

-Qy

Identify the versions of each tool used by the compiler, in
a ".ident" assembler directive in the output.

-Qn

Refrain from adding ".ident" directives to the
output file (this is the default).

-YP,dirs

Search the directories dirs, and no others, for
libraries specified with -l.

-Ym,dir

Look in the directory dir to find the M4
preprocessor. The assembler uses this option.

TMS320C3x/C4x Options

These -m options are defined for TMS320C3x/C4x implementations:

-mcpu=cpu_type

Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Supported values
for cpu_type are c30, c31, c32, c40,
and c44. The default is c40 to generate code for the
TMS320C40.

-mbig-memory

-mbig

-msmall-memory

-msmall

Generates code for the big or small memory model. The small
memory model assumed that all data fits into one 64K word page. At
run-time the data page (DP) register must be set to point to the 64K page
containing the .bss and .data program sections. The big memory model is
the default and requires reloading of the DP register for every direct
memory access.

-mbk

-mno-bk

Allow (disallow) allocation of general integer operands
into the block count register BK.

-mdb

-mno-db

Enable (disable) generation of code using decrement and
branch, DBcond(D), instructions. This is enabled by default for the C4x.
To be on the safe side, this is disabled for the C3x, since the maximum
iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than
2^{23} times on the C3x?). Note that GCC will try to reverse a loop so
that it can utilize the decrement and branch instruction, but will give up
if there is more than one memory reference in the loop. Thus a loop where
the loop counter is decremented can generate slightly more efficient code,
in cases where the RPTB instruction cannot be utilized.

-mdp-isr-reload

-mparanoid

Force the DP register to be saved on entry to an interrupt
service routine (ISR), reloaded to point to the data section, and restored
on exit from the ISR. This should not be required unless someone has
violated the small memory model by modifying the DP register, say within
an object library.

-mmpyi

-mno-mpyi

For the C3x use the 24-bit MPYI instruction for integer
multiplies instead of a library call to guarantee 32-bit results. Note
that if one of the operands is a constant, then the multiplication will be
performed using shifts and adds. If the -mmpyi option is not
specified for the C3x, then squaring operations are performed inline
instead of a library call.

-mfast-fix

-mno-fast-fix

The C3x/C4x FIX instruction to convert a floating point
value to an integer value chooses the nearest integer less than or equal
to the floating point value rather than to the nearest integer. Thus if
the floating point number is negative, the result will be incorrectly
truncated an additional code is necessary to detect and correct this case.
This option can be used to disable generation of the additional code
required to correct the result.

-mrptb

-mno-rptb

Enable (disable) generation of repeat block sequences using
the RPTB instruction for zero overhead looping. The RPTB construct is only
used for innermost loops that do not call functions or jump across the
loop boundaries. There is no advantage having nested RPTB loops due to the
overhead required to save and restore the RC, RS, and RE registers. This
is enabled by default with -O2.

-mrpts=count

-mno-rpts

Enable (disable) the use of the single instruction repeat
instruction RPTS. If a repeat block contains a single instruction, and the
loop count can be guaranteed to be less than the value count, GCC
will emit a RPTS instruction instead of a RPTB. If no value is specified,
then a RPTS will be emitted even if the loop count cannot be determined at
compile time. Note that the repeated instruction following RPTS does not
have to be reloaded from memory each iteration, thus freeing up the CPU
buses for operands. However, since interrupts are blocked by this
instruction, it is disabled by default.

-mloop-unsigned

-mno-loop-unsigned

The maximum iteration count when using RPTS and RPTB (and
DB on the C40) is 2^{31 + 1} since these instructions test if the
iteration count is negative to terminate the loop. If the iteration count
is unsigned there is a possibility than the 2^{31 + 1} maximum iteration
count may be exceeded. This switch allows an unsigned iteration
count.

-mti

Try to emit an assembler syntax that the TI assembler
(asm30) is happy with. This also enforces compatibility with the API
employed by the TI C3x C compiler. For example, long doubles are passed as
structures rather than in floating point registers.

-mregparm

-mmemparm

Generate code that uses registers (stack) for passing
arguments to functions. By default, arguments are passed in registers
where possible rather than by pushing arguments on to the stack.

-mparallel-insns

-mno-parallel-insns

Allow the generation of parallel instructions. This is
enabled by default with -O2.

-mparallel-mpy

-mno-parallel-mpy

Allow the generation of MPY⎪⎪ADD and
MPY⎪⎪SUB parallel instructions, provided
-mparallel-insns is also specified. These instructions have tight
register constraints which can pessimize the code generation of large
functions.

V850 Options

These -m options are defined for V850 implementations:

-mlong-calls

-mno-long-calls

Treat all calls as being far away (near). If calls are
assumed to be far away, the compiler will always load the functions
address up into a register, and call indirect through the pointer.

-mno-ep

-mep

Do not optimize (do optimize) basic blocks that use the
same index pointer 4 or more times to copy pointer into the "ep"
register, and use the shorter "sld" and "sst"
instructions. The -mep option is on by default if you
optimize.

-mno-prolog-function

-mprolog-function

Do not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The external
functions are slower, but use less code space if more than one function
saves the same number of registers. The -mprolog-function option is
on by default if you optimize.

-mspace

Try to make the code as small as possible. At present, this
just turns on the -mep and -mprolog-function options.

-mtda=n

Put static or global variables whose size is n bytes
or less into the tiny data area that register "ep" points to.
The tiny data area can hold up to 256 bytes in total (128 bytes for byte
references).

-msda=n

Put static or global variables whose size is n bytes
or less into the small data area that register "gp" points to.
The small data area can hold up to 64 kilobytes.

-mzda=n

Put static or global variables whose size is n bytes
or less into the first 32 kilobytes of memory.

-mv850

Specify that the target processor is the V850.

-mbig-switch

Generate code suitable for big switch tables. Use this
option only if the assembler/linker complain about out of range branches
within a switch table.

-mapp-regs

This option will cause r2 and r5 to be used in the code
generated by the compiler. This setting is the default.

-mno-app-regs

This option will cause r2 and r5 to be treated as fixed
registers.

-mv850e1

Specify that the target processor is the V850E1. The
preprocessor constants __v850e1__ and __v850e__ will be
defined if this option is used.

-mv850e

Specify that the target processor is the V850E. The
preprocessor constant __v850e__ will be defined if this option is
used.

If neither -mv850 nor -mv850e nor -mv850e1 are defined
then a default target processor will be chosen and the relevant
__v850*__ preprocessor constant will be defined.

The preprocessor constants __v850 and __v851__ are always
defined, regardless of which processor variant is the target.

-mdisable-callt

This option will suppress generation of the CALLT
instruction for the v850e and v850e1 flavors of the v850 architecture. The
default is -mno-disable-callt which allows the CALLT instruction to
be used.

VAX Options

These -m options are defined for the VAX:

-munix

Do not output certain jump instructions ("aobleq"
and so on) that the Unix assembler for the VAX cannot handle across long
ranges.

-mgnu

Do output those jump instructions, on the assumption that
you will assemble with the GNU assembler.

-mg

Output code for g-format floating point numbers instead of
d-format.

x86-64 Options

These are listed under

Xstormy16 Options

These options are defined for Xstormy16:

-msim

Choose startup files and linker script suitable for the
simulator.

Xtensa Options

These options are supported for Xtensa targets:

-mconst16

-mno-const16

Enable or disable use of "CONST16" instructions
for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled,
"CONST16" instructions are always used in place of the standard
"L32R" instructions. The use of "CONST16" is enabled
by default only if the "L32R" instruction is not available.

-mfused-madd

-mno-fused-madd

Enable or disable use of fused multiply/add and
multiply/subtract instructions in the floating-point option. This has no
effect if the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler to use
separate instructions for the multiply and add/subtract operations. This
may be desirable in some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions do not round the
intermediate result, thereby producing results with more bits of
precision than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is not
sensitive to the compiler's ability to combine multiply and add/subtract
operations.

-mtext-section-literals

-mno-text-section-literals

Control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate
section in the output file. This allows the literal pool to be placed in a
data RAM/ROM, and it also allows the linker to combine literal pools from
separate object files to remove redundant literals and improve code size.
With -mtext-section-literals, the literals are interspersed in the
text section in order to keep them as close as possible to their
references. This may be necessary for large assembly files.

-mtarget-align

-mno-target-align

When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at the expense
of some code density. The assembler attempts to widen density instructions
to align branch targets and the instructions following call instructions.
If there are not enough preceding safe density instructions to align a
target, no widening will be performed. The default is
-mtarget-align. These options do not affect the treatment of
auto-aligned instructions like "LOOP", which the assembler will
always align, either by widening density instructions or by inserting
no-op instructions.

-mlongcalls

-mno-longcalls

When this option is enabled, GCC instructs the assembler to
translate direct calls to indirect calls unless it can determine that the
target of a direct call is in the range allowed by the call instruction.
This translation typically occurs for calls to functions in other source
files. Specifically, the assembler translates a direct "CALL"
instruction into an "L32R" followed by a "CALLX"
instruction. The default is -mno-longcalls. This option should be
used in programs where the call target can potentially be out of range.
This option is implemented in the assembler, not the compiler, so the
assembly code generated by GCC will still show direct call
instructions---look at the disassembled object code to see the actual
instructions. Note that the assembler will use an indirect call for every
cross-file call, not just those that really will be out of range.

zSeries Options

These are listed under

Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code
generation.

Most of them have both positive and negative forms; the negative form of
-ffoo would be -fno-foo. In the table below, only one of the
forms is listed---the one which is not the default. You can figure out the
other form by either removing no- or adding it.

-fbounds-check

For front-ends that support it, generate additional code to
check that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran front-ends, where
this option defaults to true and false respectively.

-ftrapv

This option generates traps for signed overflow on
addition, subtraction, multiplication operations.

-fwrapv

This option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and multiplication wraps
around using twos-complement representation. This flag enables some
optimizations and disables others. This option is enabled by default for
the Java front-end, as required by the Java language specification.

-fexceptions

Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC will generate
frame unwind information for all functions, which can produce significant
data size overhead, although it does not affect execution. If you do not
specify this option, GCC will enable it by default for languages like C++
which normally require exception handling, and disable it for languages
like C that do not normally require it. However, you may need to enable
this option when compiling C code that needs to interoperate properly with
exception handlers written in C++. You may also wish to disable this
option if you are compiling older C++ programs that don't use exception
handling.

-fnon-call-exceptions

Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific runtime support that
does not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating point
instructions. It does not allow exceptions to be thrown from arbitrary
signal handlers such as "SIGALRM".

-funwind-tables

Similar to -fexceptions, except that it will just
generate any needed static data, but will not affect the generated code in
any other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your
behalf.

-fasynchronous-unwind-tables

Generate unwind table in dwarf2 format, if supported by
target machine. The table is exact at each instruction boundary, so it can
be used for stack unwinding from asynchronous events (such as debugger or
garbage collector).

-fpcc-struct-return

Return "short" "struct" and
"union" values in memory like longer ones, rather than in
registers. This convention is less efficient, but it has the advantage of
allowing intercallability between GCC-compiled files and files compiled
with other compilers, particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the
target configuration macros.

Short structures and unions are those whose size and alignment match that of
some integer type.

Warning: code compiled with the -fpcc-struct-return switch is
not binary compatible with code compiled with the
-freg-struct-return switch. Use it to conform to a non-default
application binary interface.

-freg-struct-return

Return "struct" and "union" values in
registers when possible. This is more efficient for small structures than
-fpcc-struct-return.

If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC defaults
to -fpcc-struct-return, except on targets where GCC is the
principal compiler. In those cases, we can choose the standard, and we
chose the more efficient register return alternative.

Warning: code compiled with the -freg-struct-return switch is
not binary compatible with code compiled with the
-fpcc-struct-return switch. Use it to conform to a non-default
application binary interface.

-fshort-enums

Allocate to an "enum" type only as many bytes as
it needs for the declared range of possible values. Specifically, the
"enum" type will be equivalent to the smallest integer type
which has enough room.

Warning: the -fshort-enums switch causes GCC to generate code
that is not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.

-fshort-double

Use the same size for "double" as for
"float".

Warning: the -fshort-double switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface.

-fshort-wchar

Override the underlying type for wchar_t to be
shortunsigned int instead of the default for the target.
This option is useful for building programs to run under WINE.

Warning: the -fshort-wchar switch causes GCC to generate code
that is not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.

-fno-common

In C, allocate even uninitialized global variables in the
data section of the object file, rather than generating them as common
blocks. This has the effect that if the same variable is declared (without
"extern") in two different compilations, you will get an error
when you link them. The only reason this might be useful is if you wish to
verify that the program will work on other systems which always work this
way.

-fno-ident

Ignore the #ident directive.

-finhibit-size-directive

Don't output a ".size" assembler directive, or
anything else that would cause trouble if the function is split in the
middle, and the two halves are placed at locations far apart in memory.
This option is used when compiling crtstuff.c; you should not need
to use it for anything else.

-fverbose-asm

Put extra commentary information in the generated assembly
code to make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).

-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.

-fpic

Generate position-independent code (PIC) suitable for use
in a shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table (GOT). The
dynamic loader resolves the GOT entries when the program starts (the
dynamic loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC and 32k on the m68k and
RS/6000. The 386 has no such limit.)

Position-independent code requires special support, and therefore works only
on certain machines. For the 386, GCC supports PIC for System V but not
for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.

When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 1.

-fPIC

If supported for the target machine, emit
position-independent code, suitable for dynamic linking and avoiding any
limit on the size of the global offset table. This option makes a
difference on the m68k, PowerPC and SPARC.

Position-independent code requires special support, and therefore works only
on certain machines.

When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 2.

-fpie

-fPIE

These options are similar to -fpic and -fPIC,
but generated position independent code can be only linked into
executables. Usually these options are used when -pie GCC option
will be used during linking.

-fno-jump-tables

Do not use jump tables for switch statements even where it
would be more efficient than other code generation strategies. This option
is of use in conjunction with -fpic or -fPIC for building
code which forms part of a dynamic linker and cannot reference the address
of a jump table. On some targets, jump tables do not require a GOT and
this option is not needed.

-ffixed-reg

Treat the register named reg as a fixed register;
generated code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed role).

reg must be the name of a register. The register names accepted are
machine-specific and are defined in the "REGISTER_NAMES" macro
in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way
choice.

-fcall-used-reg

Treat the register named reg as an allocable
register that is clobbered by function calls. It may be allocated for
temporaries or variables that do not live across a call. Functions
compiled this way will not save and restore the register reg.

It is an error to used this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model will produce disastrous results.

This flag does not have a negative form, because it specifies a three-way
choice.

-fcall-saved-reg

Treat the register named reg as an allocable
register saved by functions. It may be allocated even for temporaries or
variables that live across a call. Functions compiled this way will save
and restore the register reg if they use it.

It is an error to used this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model will produce disastrous results.

A different sort of disaster will result from the use of this flag for a
register in which function values may be returned.

This flag does not have a negative form, because it specifies a three-way
choice.

-fpack-struct[=n]

Without a value specified, pack all structure members
together without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this will be output potentially
unaligned at the next fitting location.

Warning: the -fpack-struct switch causes GCC to generate code
that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.

-finstrument-functions

Generate instrumentation calls for entry and exit to
functions. Just after function entry and just before function exit, the
following profiling functions will be called with the address of the
current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)

The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.

This instrumentation is also done for functions expanded inline in other
functions. The profiling calls will indicate where, conceptually, the
inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size. If you use extern inline in your C code, an addressable
version of such functions must be provided. (This is normally the case
anyways, but if you get lucky and the optimizer always expands the
functions inline, you might have gotten away without providing static
copies.)

A function may be given the attribute "no_instrument_function", in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling functions cannot
safely be called (perhaps signal handlers, if the profiling routines
generate output or allocate memory).

-fstack-check

Generate code to verify that you do not go beyond the
boundary of the stack. You should specify this flag if you are running in
an environment with multiple threads, but only rarely need to specify it
in a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the
operating system must do that. The switch causes generation of code to
ensure that the operating system sees the stack being extended.

-fstack-limit-register=reg

-fstack-limit-symbol=sym

-fno-stack-limit

Generate code to ensure that the stack does not grow beyond
a certain value, either the value of a register or the address of a
symbol. If the stack would grow beyond the value, a signal is raised. For
most targets, the signal is raised before the stack overruns the boundary,
so it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and
grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.

-fargument-alias

-fargument-noalias

-fargument-noalias-global

-fargument-noalias-anything

Specify the possible relationships among parameters and
between parameters and global data.

-fargument-alias specifies that arguments (parameters) may alias
each other and may alias global storage. -fargument-noalias
specifies that arguments do not alias each other, but may alias global
storage. -fargument-noalias-global specifies that arguments do not
alias each other and do not alias global storage.
-fargument-noalias-anything specifies that arguments do not alias
any other storage.

Each language will automatically use whatever option is required by the
language standard. You should not need to use these options yourself.

-fleading-underscore

This option and its counterpart,
-fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy
assembly code.

Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this switch.

-ftls-model=model

Alter the thread-local storage model to be used. The
model argument should be one of "global-dynamic",
"local-dynamic", "initial-exec" or
"local-exec".

The default without -fpic is "initial-exec"; with
-fpic the default is "global-dynamic".

-fvisibility=default⎪internal⎪hidden⎪protected

Set the default ELF image symbol visibility to the
specified option---all symbols will be marked with this unless overridden
within the code. Using this feature can very substantially improve linking
and load times of shared object libraries, produce more optimized code,
provide near-perfect API export and prevent symbol clashes. It is
strongly recommended that you use this in any shared objects you
distribute.

Despite the nomenclature, "default" always means public ie;
available to be linked against from outside the shared object.
"protected" and "internal" are pretty useless in
real-world usage so the only other commonly used option will be
"hidden". The default if -fvisibility isn't specified is
"default", i.e., make every symbol public---this causes the same
behavior as previous versions of GCC.

A good explanation of the benefits offered by ensuring ELF symbols have the
correct visibility is given by "How To Write Shared Libraries"
by Ulrich Drepper (which can be found at <
http://people.redhat.com/~drepper/>)---however a superior
solution made possible by this option to marking things hidden when the
default is public is to make the default hidden and mark things public.
This is the norm with DLL's on Windows and with -fvisibility=hidden
and "__attribute__ ((visibility("default")))" instead
of "__declspec(dllexport)" you get almost identical semantics
with identical syntax. This is a great boon to those working with
cross-platform projects.

For those adding visibility support to existing code, you may find
#pragma GCC visibility of use. This works by you enclosing the
declarations you wish to set visibility for with (for example) #pragma
GCC visibility push(hidden) and #pragma GCC visibility pop.
Bear in mind that symbol visibility should be viewed aspart of
the API interface contract and thus all new code should always specify
visibility when it is not the default ie; declarations only for use within
the local DSO should always be marked explicitly as hidden as so to
avoid PLT indirection overheads---making this abundantly clear also aids
readability and self-documentation of the code. Note that due to ISO C++
specification requirements, operator new and operator delete must always
be of default visibility.

Be aware that headers from outside your project, in particular system
headers and headers from any other library you use, may not be expecting
to be compiled with visibility other than the default. You may need to
explicitly say #pragma GCC visibility push(default) before
including any such headers.

extern declarations are not affected by -fvisibility, so a
lot of code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to extern functions
with no explicit visibility will use the PLT, so it is more effective to
use __attribute ((visibility)) and/or #pragma GCC visibility
to tell the compiler which extern declarations should be treated as
hidden.

Note that -fvisibility does affect C++ vague linkage entities. This
means that, for instance, an exception class that will be thrown between
DSOs must be explicitly marked with default visibility so that the
type_info nodes will be unified between the DSOs.

An overview of these techniques, their benefits and how to use them is at
< http://gcc.gnu.org/wiki/Visibility>.

This section describes several environment variables that affect how GCC
operates. Some of them work by specifying directories or prefixes to use when
searching for various kinds of files. Some are used to specify other aspects
of the compilation environment.

Note that you can also specify places to search using options such as -B,
-I and -L. These take precedence over places specified using
environment variables, which in turn take precedence over those specified by
the configuration of GCC.

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL

These environment variables control the way that GCC uses
localization information that allow GCC to work with different national
conventions. GCC inspects the locale categories LC_CTYPE and
LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation. A
typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.

The LC_CTYPE environment variable specifies character classification.
GCC uses it to determine the character boundaries in a string; this is
needed for some multibyte encodings that contain quote and escape
characters that would otherwise be interpreted as a string end or escape.

The LC_MESSAGES environment variable specifies the language to use in
diagnostic messages.

If the LC_ALL environment variable is set, it overrides the value of
LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and
LC_MESSAGES default to the value of the LANG environment
variable. If none of these variables are set, GCC defaults to traditional
C English behavior.

TMPDIR

If TMPDIR is set, it specifies the directory to use
for temporary files. GCC uses temporary files to hold the output of one
stage of compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to
use in the names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram, but you
can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an
appropriate prefix to use based on the pathname it was invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries
looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc/ where prefix is the value of
"prefix" when you ran the configure script.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are
used for linking.

In addition, the prefix is used in an unusual way in finding the directories
to search for header files. For each of the standard directories whose
name normally begins with /usr/local/lib/gcc (more precisely, with
the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning
with the specified prefix to produce an alternate directory name. Thus,
with -Bfoo/, GCC will search foo/bar where it would normally
search /usr/local/lib/bar. These alternate directories are searched
first; the standard directories come next.

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list
of directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the subprograms
using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list
of directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when searching for
special linker files, if it can't find them using GCC_EXEC_PREFIX.
Linking using GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with
-L come first).

LANG

This variable is used to pass locale information to the
compiler. One way in which this information is used is to determine the
character set to be used when character literals, string literals and
comments are parsed in C and C++. When the compiler is configured to allow
multibyte characters, the following values for LANG are
recognized:

C-JIS

Recognize JIS characters.

C-SJIS

Recognize SJIS characters.

C-EUCJP

Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler
will use mblen and mbtowc as defined by the default locale to recognize and
translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

CPATH

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by
a special character, much like PATH, in which to look for header
files. The special character, "PATH_SEPARATOR", is
target-dependent and determined at GCC build time. For Microsoft
Windows-based targets it is a semicolon, and for almost all other targets
it is a colon.

CPATH specifies a list of directories to be searched as if specified
with -I, but after any paths given with -I options on the
command line. This environment variable is used regardless of which
language is being preprocessed.

The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories to be
searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search
its current working directory. Empty elements can appear at the beginning
or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as
-I. -I/special/include.

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed by
the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target name
from the source file name. Or the value can have the form filetarget, in which case the rules are written to file
file using target as the target name.

In other words, this environment variable is equivalent to combining the
options -MM and -MF, with an optional -MT switch
too.

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT
(see above), except that system header files are not ignored, so it
implies -M rather than -MM. However, the dependence on the
main input file is omitted.

On some systems, gcc -shared needs to build
supplementary stub code for constructors to work. On multi-libbed systems,
gcc -shared must select the correct support libraries to link
against. Failing to supply the correct flags may lead to subtle defects.
Supplying them in cases where they are not necessary is innocuous.

Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation; with the Invariant Sections being
"GNU General Public License" and "Funding Free Software",
the Front-Cover texts being (a) (see below), and with the Back-Cover Texts
being (b) (see below). A copy of the license is included in the gfdl(7)
man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.